Are there (small, pod) hotels for single people in Western countries?

While this doesn't expressly address the "are there single traveler, pod-type hotels in the US?", it does offer a solution that the pod-type hotel would - lower cost.

The cost of the hotel room in a particular city (in the US, at least) is dictated a couple of factors:

1. The "chain" or brand of hotel you're booking.

   Hilton is more expensive that Motel 6. Always and forever.

2. The location of the hotel.

   Downtown is more expensive than the suburbs. Generally. Some suburbs have a large business sector and are more upscale than others.

The general concept is that business travelers aren't paying for their own room, so they're less cost sensitive (hey, the company's paying, might as well have a nice place, right?), but they're highly convenience sensitive (I don't have time to travel from the work place to a cheap place in the 'burbs, especially after wining & dining a potential client until late in the evening). Additionally, the business traveler may not have private transportation (a rental/hire car) and be fully dependent on public transport and/or simply walking from the work site to the hotel. (I was in that situation for a new job - working remote, but spent a week onsite to meet people - my hotel was literally across the street from the work site.)

However, individual travelers on vacation/visiting family/etc. are paying for their own room so they're more price sensitive and less convenience sensitive.

If you need to visit NYC, consider staying across the river in NJ. Hotel rooms will be cheaper (even for the same brand) because location, location, location! You'll pay for it, though, in additional travel time, cost and inconvenience in having to get in and out of the city each day you need to make the trip. This is exactly what my wife & I did when we visited my family in Manhattan with our young kids a decade or so ago. We stayed in Jersey and made an adventure of traveling into the city each day so it was fun for them. We took the tunnel, we took different bridges, etc. As a family on vacation, we could do that. As a business traveler, that would have been a nightmare.

Generally speaking, you're going to get the same quality and amenities by choosing a suburb Hilton v a downtown Hilton (substitute Motel 6, if desired), but you're going to pay less the further away from the city center you go.

There are single room options in regular hotels, if not all and often not many per hotel even if they do exist, so likely to be booked out when you book on short notice.

How many and how much cheaper does depend on location and the market they serve.

I find them when using a hotel booking site which allows searching for a single occupant.

Possibly not the geography you're looking for, but there is a pod hotel at the departures area of Moscow Sheremetyevo airport. It has about a dozen pods, maybe a bit more. I don't remember the terminal, though - it was an Aeroflot flight code-shared with LOT to Warsaw in June this year, if that helps. I also don't know the price, or about other airports - I didn't see any at Vnukovo, but there I only saw the arrivals area.

In any case, this is evidence that the thing you're looking for is findable outside of Japan.

Another example of pod hotels meant for single travelers/those on a budget: https://www.pandapodhotels.com/ (Vancouver, Canada), where I happened to stay recently

In Western Europe, you will find discount hotel chains like F1, Première Classe, B&B Hotels, or Ibis Budget. They do not offer capsules but rather rooms for up to 2-3 people, often en-suite. They seem to cater to a similar market, though, and easily beat your $100-150 price (typically under €50 for a room, even for single occupancy). They are common in midsize towns but they exist in major cities, too. They are usually located outside the city center and easier to access by car than public transportation.

There are also a handful of places marketed as hostels where redesigned beds turn a small dorm in a kind of mini capsule hotel, e.g. BUNK (Amsterdam) or Hosho (Paris).

Yes, many hostels in Europe have been switching over to pod hotels in recent years. There's a pretty nice one that I've seen in Tallinn, Estonia. Just search "pod" or "capsule" on a booking site and you're bound to find a few. I do not think this trend would apply in the USA, where hostels never really took off in the first place.

Belated answer, but in Denmark the Cabinn chain of hotels has a similar concept - small and cheap (starting at around 500 DKK, so around 70€) rooms that are primarily meant for sleeping in, with a small bathroom (think the kind you’d have in a mobile home but slightly bigger) attached, though of course they’re not actual capsules if that’s what you explicitly want.