Unix UIDs vs Windows SIDs - why?
While you may edit /etc/passwd (and /etc/shadow) by hand on a Unix machine, the standard way to add users is through a useradd utility (or similar) which should automatically assign the next available UID. So they should be assigned automatically rather than by the administrator. SIDs are more complicated (i.e. hierarchical) so assigning them by hand would be even more cumbersome (and besides, you cannot update the SAM database by hand anyway).
As to assigning them randomly, the SID's random part is the Machine SID, which gives SID the advantage of being unambiguous (as opposed to Unix UIDs). For example, if MACHINE1 has local user ALICE and an NTFS volume with some files owned by MACHINE1\ALICE, when you plug this volume into MACHINE2, it won't make a mistake of thinking those files are owned by some local MACHINE2 user which just happens to have the same SID (whether named ALICE or otherwise).
On Unix, if alice had UID 501 on MACHINE1, then then you plug the same volume into MACHINE2 where UID 501 belongs to bob, ls will show the files as belonging to bob (rather than to alice or even to an 'unknown UID').
UUIDs and SIDs are essentially the same thing.They're a combination of a system specific part and a timestamp, generated according to a specific algorithm (which might be different between implementations, but that's irrelevant).
Essentially they're both semi-random. Maybe some Unix admins are convinced there's some "security" reason for not handing them out or whatever, but that's nonsense.