E.g. 1 in 4 US voters will only vote for pro-life candidates. It's about the same on gun control (combined for and against). Those people basically only have 1 person they'd think of voting for.
Change would be more likely if these people had more options, so they could express the importance of their issue alongside their other views, but they can't. Being pro-gun ropes them into voting for anti-abortion candidates. Being pro-immigration ropes them into being pro-single-payer healthcare. There's no room to say "I'm pro-gun and pro-immigration"; FPTP ensures that trying to vote along those ideas goes nowhere.
Politicians don't fear for their jobs because they know that as bad as they are, people aren't going to vote against their cemented principles just to express discontent.