Because no one can predict the future, and they don't know how many resources they will need.

I mean they can just malloc when they need more, what am I missing? Unless this is about JVMs which might preallocate a ton for their heaps and not use it

Apps could, yes.

But it’s about 1000x slower to call the OS to allocate a new page, than to just use a pointer to preallocated space.