The NFL has a minimum salary of $450,000. 3-4 years in NFL at that will earn you more than the average American earns in a lifetime. That's probably a rational economic trade-off for the risk of CTE.
NFL salaries aren’t guaranteed by default. That 450k is paid weekly during the season and the player can be cut at any time. I don’t know what the median amount of money earned by NFL players is but it is likely well below 1.5 million.