Many good reasons have been covered here. The AAU circuit is breaking kids at an early age and as they age those injuries are getting reaggravated until they finally require surgery to fix. Also guys are becoming monsters physically. Look at AD's body in college and then in the pros. He now looks like he ate AD. That is alot of weight to carry around on already worn out joints, ligaments, and tendons.
However:
Here is an article from a year ago about the NBA penalizing players for not playing because missing games has become a problem.
Can't play 65 games? No MVP for you -- or All-NBA. The rule, new for 2023-24, is already starting to impact the awards race and the millions that come with it.
www.espn.com
In the 1980s, star players -- defined in the report as players who were All-Star or All-NBA selections in that current season or the prior two -- missed an average of 10.4 games per season. In the 1990s, that number was 10.6 games.
It has steadily increased since: 13.9 games in the 2000s; 17.5 games in the 2010s; 23.9 games missed per season this decade.
Are they missing games because they are legit hurt, or are they protecting their investment so to speak.
Just for some reference: Jordan played 82 games 9 times. John Stockton played every game 16 times. AC green (16) and Karl Malone (10).
Here's a study (17 years) about injury type and frequency in the NBA.
Injury patterns in elite athletes over long periods continue to evolve. The goal of this study was to review of the injuries and medical conditions afflicting athletes competing in the National Basketball Association (NBA) over a 17-year period. ...
pmc.ncbi.nlm.nih.gov
Inflammation is at the top of the list for games missed.
We as a people have a huge inflammation problem in this country. I truly believe it is at the root of most health issues. What is causing it? I could go on a diatribe but I don't really know. I suspect diet and lifestyle, but it could easily be all the chemicals / GMOs that we are inundated with.