I see what you see. As a party they believe that reducing taxes for the very rich generates growth.
There's no evidence it is true except where taxes are excruciatingly high. But they appear to believe it. And surely it does no harm to their sincere beliefs that the very rich bankroll the party.
I also agree that it is odd that some people who would benefit from a more progressive tax structure don't vote for it. That's why the GOP seeks votes among the less educated parts of US society, and also fobs them off with stuff about guns, abortions and religion. There's a bunch of gullibility out there.