I can hazard a few guesses.

One, they're losers whose high point in life was their time as an athlete in high school or college, and they learned too late that only a very, very, very select few will ever make the cut to make sports their career. So they're trying to live their dream job vicariously through their kids by pushing them into making the same mistakes they did at their age.

Two, similar to the above, except they never even made the cut into their high school team (or even the Little Leagues) and have been regretting it ever since.

Three, they think sports are the only worthy pastime that any boy or man can pursue, and since they can't play them anymore or have any kind of fun (that they don't feel ashamed of personally), they put their kids through it. But it's all ultimately a mask for how insecure and weak they really are.

Four, talented school athletes gain a lot of acclaim in their local communities, and that ends up going to the sport dads' heads, which turns them into status seekers themselves.

Five, the ultra-competitiveness of sports gets the dads into too competitive a mindset as well, to the point where they prioritize their kids' performance in a bunch of ultimately pointless games over their actual wellness.