If by 'we' you mean the English, that's easy to explain: it's because the English have a bit of an identity crisis; desperately want to celebrate their culture and history, they're just not sure they have any worth celebrating, or which won't offend anyone else. I think it says it all, when you oft. hear the question posed: 'What does it mean to be British?' = :ross: 'ing by the rest of the world. After all, if you can't even agree on what defines you, how the hell are you going to celebrate anything!?
So, instead, just latch onto other cultures' celebrations like desperate parasites. Then complain, like jealous little bitches :-({|=