I saw this in the reference today:

The datum object is the ancestor of all other data types in DM. (The only exceptions are currently /world, /client, /list, and /savefile, but those will be brought into conformance soon.)

And a forum search turned up this post (turns out people rarely use the word "conformance"), so the reference has been that way for a while.

Is it worth requesting this? It seems like if this didn't happen by now, there must be a reason why it didn't happen.

Currently you can define your own vars and procs for client objects (but they still don't inherit from /datum) and you can define procs for the world object (but it doesn't inherit from /datum), but you can't define vars for the world object and neither vars nor procs for the list and savefile types. For consistency's sake it'd be nice to be able to define world vars (global vars aren't exactly the same). It'd also be useful to be able to define list vars and procs.