IMHO the biggest advantage of a small stdlib is that breaking changes become possible in those extracted libraries. If flate2 needs to change its API for whatever reason, it can release a 2.0.
Or I guess, there can be a flate3, too.

In particular, it also becomes possible for the whole ecosystem to slowly move over to a different library, as for example happened with failure to anyhow.

If it's part of the stdlib instead, you can only do breaking changes by giving new names to function or whole modules, which will stick around forever and confuse noobies.

For example, want to copy a file in Python? Well, here's a helpful spreadsheet for all the different ways you can do that: https://stackoverflow.com/a/30359308

I mean, it's not all black-and-white. Rust has the contrary problem that the recommended library can be difficult to find out for noobies. And we may very well end up in a situation like JS, where you open a StackOverflow post and you have to look for more recent responses, because early answers are just woefully out of date.