It is a good default for lib code because it makes that code truly re-usable. The same is true for great C libraries, they let you provide a block of memory instead of calling malloc inside. The library should not get to decide if/when allocations should be done.

In Zig app code you don't have to pass allocators all over if you don't want to, that's your code so do what you want. But for libraries it's the right thing to do because it puts the caller in control.

The reason you have to pass allocators instead of making this implicit like in e.g. Odin, is because Zig just takes a firm stance against implicit logic, firmer than any other language I've ever used. The end result is that people who like Zig tend to like passing allocators around even in their own code.