> Can't it be easily added, considering an or pattern is trivially expanded to two (or more) patterns?

At least limited support has been done as a library providing a quasiquoter [0], and there is an open ticket for it as a ghc feature. [1]

[0] https://hackage.haskell.org/package/OrPatterns-0.1/docs/OrPa...

[1] https://ghc.haskell.org/trac/ghc/ticket/3919

I've been surprised/annoyed by this as well. My conclusion was that there are no practical problems that would make this hard, and it is most likely held back simply by the theoretical trivialness of it (what academic wants to work on something that doesn't make a good paper?).