One reason might be that microservice foundations tend to become frameworks rather than building blocks.

Auth is a building block you can slot into your system and the shape of the hole isn't all that complex. A framework is something you need to slot your system into, and it has a lot of twists and turns so it ends up restricting how you do all the other things.

There are lots of microservice-foundation-related building blocks. E.g. I use a library somewhere that just does circuit-breaking. I slot that into my app rather than the other way around, and if I wanted to replace it, it's a fairly isolated thing. I don't use any frameworks for it though

Frameworks also tend to grow more and more complex as they evolve. If I as a user want a different auth for example, that needs to be supported by the framework. Over time, all things become pluggable and configurable, until at some point it's complex enough that someone restarts the process, or makes an opinionated spin of the framework.