The problem I see with specifically focusing on the hip flexors is that most people have hip flexors that are way too tight as it is.
Most people do abdominal work as it is, on top of the numerous drills and other activities that work the hip flexors. Now, the only way in which I can see people not getting enough hip flexor work is through 2 scenarios:
1. They specifically try to exclude it in their program (some people have gone too far this way)
What I think is more likely is this:
2. They do current exercises with poor/inadequate technique and do not appropriately load the targeted musculature. I have been guilty of this at times, as have many others in my experience. It is easy to get more focused on doing volume and just doing the exercises versus doing them with good technique. If you are doing a variety of abdominal work and doing the movements with good technique (hanging leg raises, deadbugs, sprinter sit-ups, etc.) and do sprint drills with good technique, you will almost always have plenty of hip flexor work and additional will likely only cause problems.
I agree to an extent to you here – most athletes do have tight hip flexors. I think they would be better off working on range for hip extension. But in a well rounded programme, where hip extension/hip flexor range is worked on, I think hip flexor work does have quite a lot to offer.
With deadbugs, I quite often see people moving their supporting leg when the other leg is straight – by keeping this leg stationary they will be working their hip flexors. So correct technique would lead to working the hip flexors – same with high knee drills, providing the high knees isnt caused by lumbar movement, but actually comes from the hip, the hip flexors would be worked here.