Memory allocation can literally fail for reasons completely outside the control of the application (for example because the OS enforces a maximum virtual memory size on the application).

The fact the most allocations are fulfilled via internal pools is immaterial, at some point the allocator needs to ask the OS for more memory. This parallels the way that most I/O doesn't actually performs syscalls because of buffering.

Also allocations might end up performing arbitrary I/O indirectly if the OS needs to flush dirty pages to disk to free up memory.