I know many people are hung up on the money end of things here but there are other issues. Parker knows he has an ability to make major coin. He, like just about every 19 year old healthy young man, thinks he is indestructible and just thinks the ability to make money will always be there. But what won't always be there is the experience he can have being a kid at college growing up. He might NEED that in his life.
Yes, there's always risk of injury but time for recuperation and continuation of a basketball career are still on his side. Money sometimes, isn't everything in life and some people come to that realization a lot sooner than some.
4 years in college didn't hurt Duncan, Lillard, David West, Hibbert Brandon Roy or David Lee from earning big bucks in the NBA. 3 years in college didn't stop Noah, Curry, Horford, or Deron Williams from making huge money.
I think the whole "you have to leave early because an injury could cost you millions and millions" is basically a scare tactic, especially to those with elite type ability. The incidence of injuries that end a career for elite players is miniscule and a few million or tens of millions is still a ton of money for someone that hasn't ever earned any money in their lives.
Sure, if I was someone close to Parker or Embiid(who I have heard is also seriously contemplating staying in school) I am advising them to go pro because it is in their best financial interest, but I would understand completely if his decision was to stay in school because there will always be time to earn money later. Lots of money.