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 too agree with this, even the actions performed daily (?) as part of warm ups, many, in my observation, “go through the motions” in that the action is technically correct but way too casual (as example, are they actively putting the foot down during “A”? or is it just sort of finding its way to the floor/ground?).
However I had never really thought there was a need to do more hip flexor work until I ran into athletes with a series of similar injuries a couple of years ago, so I learned a lot at that time about that area generally and in many of those injury cases it was not truly strength related per se, there were some imbalances that were important in the causal relationships but those were worked on after the ROM issues were fixed, as in speedfreak1 statement.
Don’t under-estimate what “simple” actions, say planks/pillars in three positions w leg raises can have when mixed with a good mobility routine (say hurdle walkover series).