Multiple reasons, but it starts with terrible education. They don't know how to verify information at best, and at worst don't want to, they believe they win when they piss off someone they don't like. This means the information they get on current topics is generally something pushed by someone either paying an algorithm to target them or by things like AstroTurfing.
There's a lot of value for the hegemony class in getting people to not care about the climate, endless war, genocide, and human rights in general.
And then you have the out of touch authoritarian liberals/neoliberals that control the liberal side of the mainstream media.
I used to think the same thing and there are people that change their minds. But for a lot of people in the trump cult if something doesn't work out well they will find a way to blame it on the libs. They are quite practiced in circular logic thanks to their religion.