When liberals keep selling far-right positions back to the masses, why would the masses bother voting for the people who only just started saying those things were issues, when they could listen to the people who've been saying it the whole time?

The reason why fascism is capitalism in decline. Capital captured politicians can't reverse course on policies that increase suffering for the benefit of profit, so they have no answers to the growing problems of profit-seeking without end. Fascists don't have answers either, but they don't have to worry about things like reality or political possibility when selling their version of the future. They just agitate, villianize outgroups, and win when liberals/centrists try to repeat their politics in an effort to chase the Overton window.