Because they’ve been told to think that. Trump made liking Russia and hating its enemies part of his campaign because he was stupid enough to think they were friends, so his followers adopted that.

He made anti-vaxxerism part of his administration because his negligence, incompetence and stupidly resulted in a terrible pandemic, and all he cared about was how it made HIM look. So his followers adopted that.

He claimed the MSM is the "enemy of the people" and everything they said was “fake news” because they asked the tough questions, called him on his lies, and wouldn’t be his echo-chamber (unlike FoxNews). So they adopted that.

He told them the 2020 election was rigged because he lost, in spite of his best efforts to rig it. He regularly and preemptively claimed that the 2016 election and 2020 election would be rigged because he anticipated losing both times. So they adopted that (and committed insurrection).

If MAGA conservatives have proven anything, it's their infinite capacity to do and think whatever they are told to. They are incapable of informing themselves or even thinking for themselves. Everything to them is whatever Trump, alt-right media, and Russian internet trolls tell them to believe. Everything else, they've been trained to reject as "fake news."

And if you doubt that, consider the fact that he convinced people to drink bleach, take horse medicine, reject life-saving vaccines, bankroll his retirement, and commit violent treason. People are dead for having listened to him, and somehow, they still haven't learned!