> 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