I can relate. Since I became more aware that I had health anxiety, I've read a lot of theories about why people have it. It's different for everyone of course.
However, the reason why people have chronic health anxiety is that something is not right. I don't mean it's whatever disease we're afraid of if we clearly don't have it, it's something else in our lives that isn't working.
For some people it could be a biochemical problem, hormonal imbalance, or just more of an awareness than others typically have. For others, it can come from some kind of trauma they haven't been able to cope with, or it could be a symptom of something else not being right about their everyday lives. Like feeling overworked, under-appreciated, trapped. Those kinds of things can manifest as anxiety.
The point is, whatever the reason anybody has anxiety, it isn't a choice, it isn't something that can be easily turned off. So don't blame yourself for being scared, but do try to find ways to treat your anxiety. If two doctors are telling you you're alright, I know it's still possible something was overlooked, but it is worth looking in to every option possible to feel better, and that might mean help with mental health first.