as others have said, each airline serves a different market and it's not in IAG's interest to have them all offer the same. there are lots of examples in other industries (RBS Group which owns RBS, Natwest, Coutts Brands, Lloyds Banking Group which doesn't standardise offering between Halifax and Lloyds Bank and Bank of Scotland, etc, etc etc.)
so why would / should Vuelling or Aer Lingus offer same benefits / be part of OW? yes, you / we would benefit but not the average flyer that uses them.
on that latter point, KIV that most people outside of Flyertalk will not know of the existence of IAG (which, like Unilever, is not a customer facing brand) and I'd bet the majority of their pax and the general public would not in their wildest dream think there's any connection between Vuelling and Aer Lingus.