I think that's because the proper way to accomplish something like that in SQL is via specification. I do agree with you, though, postgres added enums (another feature that it's not strictly needed in SQL, but nice-to-have) but not something like that.

It's possible to emulate union types in SQL with triggers (i.e. check that the field A and B cannot be NULL at the same time, but it does not work if NULL is be a possible value.)

An aside: I wish some relation database adopted sum types: I didn't thought about the implications, but doing 'create table foo ( bar Maybe integer );' and then 'select Some bar ...' would be cool (and maybe a cleaner way to work with NULL.)