At least with ELF, shared libraries are organized into segments and you really only have a handful of them. The information about where individual function boundaries are has been thrown away and in principle this can matter, e.g. if cold code was moved away from hot code. Therefore, the linker can't just split the segments apart.

As for your instinct, it'd be wildly wrong depending on the use case. Maybe if you only link with libc, sure. But now consider linking against something like a GUI framework...