I do not understand how anyone can question evolution.
It comes from a place of ignorance.
In America, in particular, there is also a lot of deliberate misinformation put out about it by those with religious agendas. That has come about because of the ongoing attempts by religious fundamentalists to find ways around the separation of church and state, particularly when it comes to getting their religious beliefs taught in schools.
Putting aside the deliberate liars, it almost inevitably turns out that those who question evolution (i.e. its basics) just don't understand the basics of the theory. In some cases, they are simply ignorant. In other cases, they have been led to believe that the theory of evolution is an evil thing that threatens their religion, so they should make sure they never put themselves in a position of finding out anything real about it. Those people believe their immortal souls are put at risk by exposure to this dangerous idea.
I certainly can understand.
Those who question science in general probably have been told since they could listen that the bible is the word of God and is the absolute truth.
Added to that, there's also a growing distrust of perceived authorities in certain quarters. Scientists can be perceived as aloof protectors of arcane knowledge, who are overconfident in their pronouncements, especially on matters that impact the competing authority structures of religion.
In America, there is also quite a lot of deliberate misinformation about science put out for political reasons that are not necessarily religious. Take the topic of climate change, for example. Undermining the trustworthiness of science in one area tends to reinforce the idea that science in other areas is not to be trusted.
Both the anti-evolution and the climate change denial anti-science movements are well funded and determined. It is little wonder that Americans are outliers among first-world nations in terms of the proportion of the population who say they don't accept evolution.
In short, it comes down to lack of education, bad education and, in a minority of cases, bad faith.