Drive letters in general are a legacy holdover from MS-DOS. The Windows NT kernel doesn't use them. It is a user-space DLL that maps the kernel's single tree into drive letters.

All other operating systems use a single tree with mount points instead. Windows supports mount points as well, but its default behavior is to assign a drive letter.

Drive letters are still useful, though, if you have multiple drives and

• they're removable drives (optical disc drives, USB drives, etc), or
• they're internal, but you want to keep them separate (i.e. not RAID).

Other platforms deal with this by reserving a subtree for mount points (/media on Linux, /Volumes on macOS), which is functionally equivalent to drive letters. This does have the advantage that mounted volumes are identified by a name rather than just a single letter, but on the other hand, the path to the mounted volume is longer and less convenient to type.