Why isn't there a Wine-like compatibility layer for UWP apps?

Probably some of both. UWP never became the dominant format for Windows software, and interest in it has faded. That leaves a relatively small pool of apps to justify a large amount of compatibility work.

Wine also represents decades of accumulated development. Making a comparable layer for another set of APIs and app infrastructure would be a big project, not a quick addition.

It isn't necessarily a clean split between Wine and a separate UWP layer. Wine has some basic UWP-related support, but that doesn't mean a particular UWP app will install or run. The missing pieces depend on the app and how it's distributed.

Distribution is another complication. Some apps from Microsoft's store are encrypted, so implementing the APIs alone may not get you a usable app package. There can be legal questions around bypassing encryption too, and those vary by jurisdiction.

For games, the interesting question may be less about UWP itself and more about whether Game Pass titles could be installed and launched. Supporting that could involve store and service requirements beyond whatever compatibility an individual game needs.