You forget Kevin Garnett, the player who started the modern 'straight out of high school' craze.
The problem is that whether or not a player is physically ready for the NBA at age 18, he's a legal adult and is entitled to try if he wishes.
The NHL went through a phase in the early 1980s where a lot of 18 and 19-year old players made the league at the same time. Guys like Wayne Gretzky, Ray Bourque, Paul Coffey and Steve Yzerman all started their careers at age 18 or 19 during that time and the 21-team NHL went through one of its best phases.
The Major Jrs makes hockey very different when it comes to drafting young guys. It’s basically minor league hockey, but high school kids play in it, they get a pretty brutal schedule and still have to go to school. In a lot of ways it’s college basketball for hockey.
A league has a right to say that players of a certain age lack the physical and mental maturity they desire to project the image they wish. There was a serious drop in the quality of NBA play and an uptick in off court legal issues when they were drafting high school kids. I’m not sure play quality has gotten fixed, but the legal issues have. Might not be related to the age of players being drafted, many explanations are possible. But it’s the league’s right to decide they shouldn’t be bringing 18 year olds into a sport often described as the world’s largest traveling brothel.