Why don't airlines have backup planes just in case of an emergency?

Because it would be ridiculously expensive.

Not only would it require a massive increase in the number of planes an airline has to purchase and maintain, it would not even garantee that there would always be a spare since there are many different types of airplanes that serve different purposes. It would be impossible to have one of each on standby in every airport.

Airlines therefore chose to live with the cost (and reputation hit) of not having immediate spares, even if that means compensating passengers or even losing customers.

Small caveat: Airlines will usually have some aircraft on standby for a variety of reasons. For example: at their main hub, for maintenance, etc. My answer is targeted towards airlines not having enough spares to cover for every flight delay around the world.

Look at a typical airline company on Wikipedia. Let's take KLM for example:

• Fleet size: 117
• Destinations: 138

As you can see, in order to have a spare plane on every flight, KLM would need to buy more spare planes than they have in the actual fleet, meaning their ticket prices would have to double in order to cover the expenses. Since people are not ready to pay twice the price for the comfort of saving a couple of hours in rare cases of cancelled flights, this is not happening.

They usually do? We just don't notice when backup gets used.

They have some planes to spare and they can wet lease planes (including crew) from other companies. They can also redirect passengers to another airlines.

Cancelling flight costs money (and reputation losses) and getting spare plane also costs money. They just have to choose what's cheaper in a given situation.

From my personal experience, I had my tickets transferred to another (more expensive) airline, had older/unusual plane for boarding instead of a regular one.

Most larger airlines do have aircraft in reserve for just such situations. Most often, they are aircraft that just completed a maintenance cycle and are not scheduled to begin a normal flight sequence for 1 or 2 days.

The problem from the passenger perspective is that these reserve aircraft are at a hub or maintenance facility, not the station with the out or service aircraft. So, even if they did use a spare, it has to get there first.

Another option the airline has at larger stations is swapping in another active aircraft that's not scheduled to fly until much later.

It would be prohibitively expensive to have spare aircraft at every station. Prohibitive for the passengers.

The reason air travel is as cheap as it is, is due to the planes being used for as many hours a day as possible. If an airline had one plane spare that would 100,000,000's of pounds sitting doing nothing when it could be earning them money by flying, if that plane could earn them £10,000 a day if used but if used as a spare it would save them £100,000 when used but is only used once a month then using it is the cheaper option.

It depends where you are in the airline's route network. If it's a hub airport then odds are better that there will be one or more comparable spare planes available at the beginning/end of a maintenance cycle. This probably means a delay of 3-6 hours though still because it's often not quick to bring something into service and takes a while for that decision to be made to begin with.

If you're at an airport where there's one (or fewer) daily flights to/from a hub that turnaround and head straight back on landing (a spoke in the hub/spoke model) then it makes no sense to have spares available, instead the airline will have minimal facilities available and the backup plan will almost certainly be "fly a spare aircraft out there" on major failure. Delays are likely to be much longer in that scenario.

Airline scheduling is a horrendously difficult task. Airlines have to strike a balance between multiple priorities, such as minimizing cost, maximizing revenue, having just enough passenger capacity, satisfying crew requirements, and not exceeding the allotted airport space. Airlines do have some spare capacity to account for scheduled maintenance, but there is no way to swap aircraft at a moment's notice for unforeseen breakdowns while meeting all of the other economic demands.

Keep in mind that airlines try hard to minimize turnaround time at the gate, because every minute a plane spends on the ground costs them money. To maintain a spare idle plane at every airport is simply not realistic. On top of that, aircraft differ in capabilities, so they are not freely interchangeable. Obviously, larger planes are required for high-traffic routes, and smaller planes are required for flying to regional airports. Less obviously, there are subtle differences as well: in August 2015, American Airlines mistakenly used a non-ETOPS-certified plane to fly to Hawaii, and got in trouble for it.

Not only do you have different types of aircraft, you also have to have suitably matched crews available for them. In particular, the pilots have to have the appropriate type rating — a 767 pilot is not allowed to fly a 737.

Given all those considerations, it is generally much more economical to rebook passengers on alternate flights than to keep spare planes around.

Previous answers cover "spare" aircraft which are in active service but have been rotated out for repairs. In addition, there are desert storage "boneyards" for aircraft which are being taken out of service longer-term. Many (most?) of the aircraft in boneyards have been permanently decommissioned, and are being held there to be broken for spares. Sometimes though these aircraft are simply being stored whilst demand for flights is low, and they will be returned to service in the future when demand increases.

It should be clear though that getting one of these out of storage is not simply a matter of fuelling it and away you go!

There is not enough aircraft parking for all aircraft to park: in disasters which close large numbers of aircraft, they have to use runways and such to store the planes.

Keeping an idle aircraft for every one aloft would be infeasible because there'd be nowhere to store them.

But the airlines could get together and have a shared plane or two fueled at each major airport, using something like collective risk management/insurance schemes.

But then... who'd fly them? They'd also need to keep an airline crew on call at those airports. There'd be a call-in time, so an immediate swap-out would not be feasible, unless the on-call crew were in the airport at all times.

Something like this appears feasible, at least at first glance. That it's not done suggests that other types of damage control are considered more effective.