Ok, I'll bite. The answer is because it's not practical. One cannot simply slow their pace down and do 10,000 meters worth of steepling. Eventually, it would cease to be a race and "racers" would literally come to a stop at every hurdle and step over it. Over the course of a 3k steeple, even the best steeplers undergo some degree of degradation in their hurdling form, albeit very minor in elite steeplers. The only runners who could even complete such an event would be the most elite runners would be able to finish still hurdling every barrier. At that level, what incentive would meet directors have to add such an insane event which would surely take upwards of 30 minutes into meet schedules they are already trying to keep at a minimum? Without a financial incentive, those capable of completing such an event well would never do one. Now, anybody is welcome to set up barriers and go for a 10k steeple whenever they please, but it's practicality as a contested event at a competitive track meet is nonexistent. You could really say the same thing about the marathon, "Why is the marathon only 26 miles, why don't they have longer marathons." The answer, of course, is that they do, but that they're not taken very seriously among the mainstream running community.
Also, I appreciate your questioning of my credentials. Though I don't feel adequately incentivised to lay out my resume for you, but rest assured, I do know a few things about racing on the track and in the marathon.