I don't think anyone realizes how much most high school athletes would despise a 5k on the track. That is a miserable, miserable event, and most of them already hate the 3200 and only do that if their coach makes them. Maybe the most serious 5% of runners would do it, but they still wouldn't like it.
Moreover, there is already an entire season of running nothing but 5ks. Some athletes would then have another season of only 5ks. That would make distance running, one of the most innately boring sports, even more boring as the athletes would have less variety.
Who gives a sh!t about getting ready for college? That is relevant for less than 1% of the athletes. Who cares about doing more aerobic training to get ready for later in life? Only a few people of the top athletes in the country need to worry about that, and they can take care of themselves. Most kids don't want their training to be even more long, slow, repetitive miles.
Furthermore, the 5k is extremely boring to watch, and with one 5k for each gender it could easily take 50 minutes to get it over with, boring all of the spectators.
Also, why does it matter that distance is less important to team scoring? By that logic should there be many more throws and jumps events, with all sorts of different implements and techniques? There are an equal number of individual sprint and distance individual events, three apiece. There are more sprint relays because: 1. They are way more exciting to watch, 2. They take up much less time, 3. On most high school teams, there are significantly more sprinters than distance runners.