Sure, they could try that. But companies/rich people typically only invest money in things that give them a return. Rich people are generally rich because they don't just throw million of dollars of away on items which give them no return.
Investing that kind of money in high school kids is risky business. Every year, there are plenty of 5* talents that are total busts. So, where is the return? It's a high risk, relatively low reward situation in that example. There are other things to consider with paying high school players, such as marketability. Most of the time a tiny percent of the population has even heard of this person. Not very smart to invest in someone who is unproven, unknown, etc... When Sam Bradford was in high school, the only people who knew who Sam Bradford was were a very small percentage of Oklahomans and random people who saw him on Rivals. We are talking about 0.00001% of the population. Who would invest in this guy? Nobody would.
Professional athletes and even college athletes are a much lower risk and a much higher return for that kind of thing. They have already proven themselves, they are already famous, well known in the national media, etc.
Johnny Manziel wouldn't have made anything coming out of high school. He only became "famous" after he established himself at A&M. He was a 3* recruit with nothing but A&M and mid-major offers (per rivals).
I don't think this would be an issue, in the way you are describing. The people who would be making money would be people who are already established, famous, college athletes.