From what glibc developers have told me, glibc supports a huge number of systems, and a lot of the "bloat" in glibc comes from supporting various architectures and operating systems accumulated over the last 30 years. In general, code is not deposited in glibc at random, so somebody did see a good reason to do whatever it is at the time it was done. Musl is nice but it's not a strict replacement for glibc in all cases, and the musl developers seem to be pretty honest about that, for example here: https://wiki.musl-libc.org/functional-differences-from-glibc...

Also in my experience, the attack surface cannot be strictly reduced by removing features -- if users want a specific feature, they will just get it from something else, so at some point one needs to have code on the machine somewhere that does it.