There are a few reasons why context switching in user mode could be faster, but that has little if anything to do with the performance benefits of usermode threads. The performance benefit of usermode threads is a result of their quantity, due to Little's law. They're not "faster", just more numerous, and that's what you need for higher throughput. More precisely, OS threads, because of their scarcity, introduce an artificial bound on throughput that's lower than what the hardware can support, and usermode threads remove that bound.

More here: https://inside.java/2020/08/07/loom-performance/

As to why it's hard for the OS to allow that many threads, the OS would need to keep thread stacks small and resizable, and that is hard to do if you don't know the specifics of how the language uses the stack. For example, to accommodate low-level languages that allow pointers into the stack you would need to manipulate virtual memory (to keep addresses valid), but that only works at a page granularity, or to introduce split stacks, which would require a new kind of ABI known to compilers (and would probably have a cost to performance).