There are several reasons:
Localization of the assets:
You can't place a game in the UK if a car will be shown. Every Daz car has the steering wheel on the left.
Most electrical sockets in assets are American ones, so that discards Europe as a setting if you don't want to get complaints about the sockets being incorrect. Yes, there are some assets with (continental) European sockets, but you don't want to limit yourself to perhaps 10-15% of available Daz houses/bars/cafes/etc.
Related to the above point, is the perceived housing distribution: Most people in Europe (living in a medium-sized or bigger city) live in an apartment or small house (no pool, no garden to write home about, etc.). Hollywood has "sold" the idea that every middle-class American can live in a big house with huge rooms, a pool, and a big garden. You don't want your characters having sex in a 3mx3m room if you want to move the camera around. The MC's room is 19'3"x23', 442 sqft (5.86x7m, 41m2). There are smaller apartments in some cities in Europe.
Europe has a history. If you set your game in Paris, sooner or later you should show the Eiffel Tower, the Chams Elysees, etc. Same with London (Westminster Abbey, the Houses of Parliament), Rome (Colosseum, Fontana di Trevi), or wherever. Most small towns in Europe have historic landmarks and almost none are available as Daz assets. It's easier to "invent" a small town in the US than one in Europe.
Market: As you can see in the table below, most of my income comes from US players (it shows money percentage, not percentage of players, in Subscribestar in the last year, easier to gather that info than on Patreon)
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It's not only a population thing; American players spend more per million inhabitants than any other country (except Australia and New Zealand), followed closely by Germany.
If a market provides you with 65% of your income, has the asset advantage, and also it's easier to invent a setting, it's a justifiable choice to set your game in that market.