Supply and demand. Your employer could find thousands of people to replace what you do. In pro sports you can't find another McDavid, Lebron, Trout, etc.
These guys are the top 1% of what they do. The literal best of the best.
Anyone who is in the top 1% of what they do whether it be in the medical field, engineering, art, music, etc will be highly compensated. Do you think the pay of the top surgeons and doctors in the world are equivalent to your run of the mill family doctor? Do you think soundcloud rappers should make the same as Drake? They both make music.
I will also point out that in entertainment, more than any other field, that these guys take home a lot less of their money than a normal person.
I think the top athletes in the world make between 90 mill and 110 mill last year. I believe most people probably take home 50-60% of their salaries, but these guys probably don't get more than 40%. That is still a crap load of money, and they are definitely not the only ones who help them earn that money (i.e. there are certain guys, who are not easy to find/replace, who help them make a lot of their money), but at least with athletes the quality of their work is not really subjective... The money they generate and the quality of their work is fairly objective. You typically are not making that money unless you are really good at your sport, and it is because of this that you are generating money for teams.
With other entertainment sectors, like music and even acting, it is a lot more subjective. There are a lot of time when brand name and marketing drives sales more than anything the artist themselves does. The Rock was the highest paid actor last year (making almost as much as LeBron) and quite frankly he is not the best actor in the world. His stuff is okay, but I would not be saying that anything The Rock does is worth 90 million dollars a year. People really like him and his stuff though I guess. Same goes for Taylor Swift, Drake, etc. Some people think that Taylor Swift's/Drake/The Rock's stuff sucks, or is not nearly as good as it is made out to be, but you can't really say that LeBron/McDavid/Trout are not some of the better players out there, and it is more or less laid out how much players make relative to the revenue they generate.
The way I look at it is that you need to be very lucky in the entertainment industry. You still need to work really hard and be talented, but there is just so much that is out of your control that hard work and talent is not enough to be successful (or at least super successful)... So you will find really talented and hard working entertainers who make less money than they should, and others who make a ridiculous amount more than they should. You need a lot of luck in every other industry too, but at least a lot more things are within your control and often not as subjective, so hard work and talent usually can get you a higher baseline of success than in entertainment. Your ceiling is often still very much out of your control though.