One thing is that commercial libraries are often built from houndreds or thousands of small source files. So the library contains huge numbers of small object files containing individual functions.

The linker then just picks up the functions it needs, instead of picking up a huge package just to get access to a tiny function.

But this is also a question of tools. A "standard" C linker can only link full object files. Some architectures or some compilers/linkers have extensions where the linker knows how to cut an object file into smaller blocks.