I think its more accurate and informative to say that track is one of the only sports where being a "pro" mean relatively little in terms of payment and support.
If you're in college and you're a world class runner - say, a Nico Young - you have EVERYTHING you could possibly ask for. A full-time training staff, free sports-related healthcare, a training venue, gym, recovery tools, room and board likely paid for, free education and also most likely getting paid money on top of that. You go to class an hour each day and that is your only real obligation beyond training, eating, and sleeping.
WHY would you leave that behind?? For the vast majority of runners, it makes far more sense to stay in college and enjoy that lifestyle as opposed to going pro and taking on all those costs yourself. Because the truth is, sponsors are not putting enough money into this sport to make going pro - I mean REALLY pro - feasible for the number of high level runners there are.
This may be a hot take, but the flipside of this is that there are simply too many people in the sport at a high level trying to "make it." The money that is available gets spread too thin. A potential but controversial solution is to whittle down the number of events which make the fields denser and more competitive. But that's a half baked thought and I'm not 100% sure what the best way to implement it would be.