For one, that was a long time ago. Seniors rarely go high in the draft these days. For every Kenyon, or Buddy Hield, there's a lot more like Paul Harris that missed out on their payday because they had to go to school. Paul probably doesn't make it long, but at least would have got a shot, and over a million in the bank. Then, he could have went elsewhere, and kept making money as he did anyway. Just missed out on that original chunk of money, and years of pay in a career that has a short shelf life.
No need for Porter Jr. and many others to go to school except get hurt, or show they aren't as good as advertised.
CFB makes sense as a kid going straight to the NFL may get mauled. MLB they have a choice. The NBA forces kids to potentially ruin their stock, or career via injury or underperforming. Many aren't going to school to learn. Those who can't qualify get screwed, and may end up missing out on their dream.