It is really no different than than thousands of other jobs with a much higher risk than an NFL player has. Miners, loggers, firemen, policemen, farmers have much greater risks of death, and many are physically as broken down in the end as are NFL players.
Generally, a worker is paid what is necessary to find qualified people to do the job. Even though it is really no different than other jobs, major pro-sports have gotten themselves in trouble by defining a sort of "partnership" where labor (players) are treated like a co-owner. Sometimes that works in the regular world too, and sometimes it breaks down.