It’s because there are truly 40% of people who are stupid. Not just dumb, but actively, incredibly, unable to think for themselves stupid.

Some of them aren't necessarily stupid. They just have different priorities and beliefs about things than you and I do.

Imagine you had a Democratic President that was as terrible as Donald Trump, but had done two things in his time: i) appointed enough liberal justices to the Supreme Court to overturn Citizens United and Heller and Dobbs; and ii) adopted Medicare for All. Terrible, terrible person - venal and dumb and corrupt - but had managed to do those two things. That President would have a non-trivial amount of people who approved of his job performance. Partially because he was not a Republican - and is therefore not doing any of the things a Republican would do, and Democrats greatly approve of Presidents not being Republican. But also because he had done two things that Democrats really, really, really wanted done.

I bet that person would have a non-trivial "floor" on their approval rating. Maybe not 40%. But probably not below 30%. Not because that 30% was stupid, but because he had delivered something they wanted so much that they would look past the other horrors and give him a net passing grade.

Trump's done three things that his base loves. He gave them the Supreme Court they wanted (admittedly in his first term, but still), he stopped border crossings the way they wanted (the big policy thing), and he's not doing all the stuff that a Democratic President would do. For a big chunk of the GOP base, that's enough for them to approve of his job performance.