There's a few reasons. From a substack I wrote a while back:

A 2014 study analyzed the data of more than 7400 people and found that “perceptions of preborn humanness explained very little of right–left differences in abortion support, and the association between preborn humanness perceptions and abortion opposition was no stronger for those on the political right (vs. left).”4

By contrast, a 2017 study found that sexism accounted for 30% to 70% of the left-right difference in abortion stance even after controlling for other relevant variables.5 An earlier study found that authoritarianism had a significant correlation with an anti-abortion position and aggression towards women,6 and a 2019 study found that right-wing authoritarianism had a significant correlation with anti-abortion stigma both before and after controlling for other variables.7

A Pew poll also found that 33% of Americans simultaneously believe that a fetus is a person with rights and that the decision to abort should be up to the woman, so the idea that life begins at conception explains only some of the difference in abortion opinion, with it mostly being determined by factors such as sexism, religiosity and sex. It should go without saying that Republican politicians are mostly conservative religious men, and there's far less diversity of demographics and opinions among them compared to their constituents, and the Republican party takes sexual abuse far less seriously than Democrats.

In that same Pew poll they also found that while a significant chunk of Republicans believe that abortion should be legal in all or most cases, a majority (60%) still believed that abortion should stay illegal in most or all cases. So Republican politicians may also believe that this is still a winning issue for them in the culture war (although the "red wave" that never was may suggest otherwise).

The GOP also gets a lot of donations from anti-LGBTQ and anti-abortion religious groups, so they are undoubtedly motivated to please said donators to keep the money coming, even at the expense of their voters.