Because the party is run by the Rockefeller/Romney wing who agree with the Democrats on social and cultural issues. They’re the same liberals but they’re run by a different set of wealthy donors who insist they keep taxes low.
I think you’re right. The GOP sells out social conservatives every chance they get. For example, you know they’ve wanted an immigration amnesty for years. They’re not even all that fiscally conservative. Standing firm on tax increases while doing absolutely nothing to reform spending or slow the accumulation of debt is NOT fiscal conservatism.