This is actually something I've always wondered myself.
Personally, I don't think most players gain much, if anything, from garbage time. It's not close to real NBA level basketball, the offensive sets are basically running at half speed with a packed in defense also running at half speed. It's basically the last pickup game of the day at your open gym after all the good players have already decided to call it a day. And unless this is your first time playing, you're kind of embarrassed you didn't get picked to play in any of the good games with the good players, and you're stuck playing with somebody's little brother who just entered junior high and the foreign guy who just discovered basketball last week. Sure maybe a decent player or two is playing, but they're exhausted from playing in the real games and start taking circus shots (not that this has ever happened to me

).
While I understand not being able to throw guys in for long minutes for fear of losing the game, and also you don't want to mess with the rhythm of the guys who actually get minutes, there's plenty of times I'm watching games and I think, man this seems like a perfect time to give one inexperienced guy some burn. It's a 4-5+ possession game at the end of the half or 4Q (or any time in between even). What happens if you throw one of Mickey or Rozier or Young in for 2 minutes (just one of them, not all of them). Let one of the young guys get a couple high level NBA possessions in instead of several D-league quality possessions in at the end of a blowout, while also possibly letting them feel like they've contributed to a win by playing meaningful (but brief) minutes.
So yes, I wonder why it seems like more young guys aren't blended in during meaningful minutes, even if just for short stretches.