Why not space out memory allocations?

Question

In ext4 file system, the files are spaced out as far apart as reasonably possible to allow for efficient reallocation. Why do we not do this in memory?

Why not allocate one memory as page 20, and the next large allocation as page 100 to allow for excess expansion before and after the current allocation? I feel that makes sense for large frequently changing-in-size buffers. For smaller buffers, doing so would probably be a drain on memory because we have to allocate each page for a small amount of bytes(but perhaps do it within a block too in the malloc impl). This would also provide greater memory corruption prevention, allowing segfaults(or windows equivalent) to happen quicker(specifically for larger buffers).

Why don't we do this? I get it on modern 32-bit systems because of limited address space, but why not on 64-bit?