And there are also some mysteries where the protagonist's lack of formal authority to ask questions is clearly part of the story line - where the protagonist has to cajole or finagle others into answering, or find info out in other ways, like by surveillance. And in some cases, characters will start out talking to the protagonist, but then balk at some point and quit talking, which actually seems to me like something that might happen a lot.
In general, though, I agree with @issybird that I think a lot of people are a lot more willing to say a lot more than one might expect.
I once participated in a workshop for entrepreneurs that put a big emphasis on getting primary marketing input, just by talking to potential customers. The workshop instructors told us that if you just ask, a lot of the time you get answers, and then they sent us out to do that. I was amazed, but it was really true, if you frame yourself as asking for help, and indicate the value that you are going to put on someone's input, and really listen, lots of people will try to help you out. Granted, we were trying to find out if people would like a product, and not gather info about a crime
, but still, I was truly amazed by the amount of time people would spend with us, and how much they would tell us.
So, I guess every now and then, I have to suspend disbelief, but a lot less than I used to
.