Why are round-trip car rentals much cheaper than one-way car rentals?

So, first off, this isn't always the case. Point to point rentals can be cheaper in some circumstance. Especially if you're taking a car from an area with low demand to a return lot with high demand. You can get some great deals this way seasonally, but they're hard to shop for.

That said, the reason it's generally cheaper to return the car to where you got it is because if you don't, somebody else will generally have to. Point to point rentals are rare, so when they happen, it throws off the very carefully managed inventories of multiple rental lots. Suddenly, one lot is short a car, and another lot has a surplus. In order to rebalance things, they either need to hope somebody wants to reverse your itinerary (unlikely), incentivize somebody to do so and lose money offering them a bargain, or pay an employee to transport the vehicle.

This really depends on the franchise, licensing and ownership in that area.

In Germany, I rented a car in Frankfurt and returned it to Munich, no extra cost (national owner, national registration).

In Calgary, I can rent a car at the airport and return it at any city location, no extra cost (same franchise).

If I rent a car in Calgary and return it to Edmonton, I pay a one-way fee as someone has to drive the car back (different franchise, same registration). They might get lucky and have a one-way renter drive it back.

If I rent a car in Calgary and return it in Toronto, the agent in Calgary simply sells the car to the Toronto location. It's cheaper than returning it. (Different franchise, regional registration. Ontario franchises cannot rent an Alberta-registered vehicle.)

I asked the counter staff why I have two keys on a steel-cable (non-separable) ring, the second key is clearly not a spare. One-way sales happen often enough that they want all the car's hardware to travel with it.

It spares the agency the expense of driving the car back to the original agent or otherwise managing the inventory - rental car employees spend a lot of time driving cars from one agent to the other and then someone has to drive the employe

There is another factor, often the best deals are from small companies that just have one branch. Clearly these companies don’t offer the option of returning the car elsewhere.

So as to complete with the small companies and still make a nice margin, the large companies “over price” the add-ons that the small companies can not provide.