In college, a player gets a lot more practice time. The practice to game ratio is higher. In the nba, a team can go a week or so without practice.
NBA success comes from talent, hard work, size and confidence. In college, a player can feel more encouraged about learning post moves, off hand stuff, other fundamentals, when playing against lesser competition. In the pro's you rarley see much significant low post development.
But i agree that if you are projected to be picked in the lottery, you should probably declare for the draft. The lure of financial security is too high. At that point you just hope the raw player is playing with good teammates and a good coach in a well run organization.
While it is true that there is a higher practice time to game time ratio in college, the NBA season is longer and NBA players can work on their game 24/7 year round. They do not have to go to classes. They are also surrounded by the best basketball minds in the NBA.
I personally feel that if guys enter the league to early, it won't matter much. Sure, they might not be ready for NBA talent levels, but there are plenty of ways to develop in the NBA or outside the NBA if they can't stick in the NBA. Look at how many players came from foreign leagues to become perennial all stars. There are second chances. We have guys in the NBA who went NBA-Europe-NBA.
Another year can often just expose the lower than originally expected upside of the player in question. Players should strike while the iron is hot. Every sports league agrees to get guys under contract who might not be ready for prime time. How many excellent QBs stood around with a clipboard their rookie seasons? Quite a few.
I think the development aspect of college basketball is overstated. Coaches in the NCAA's top programs need to excel in recruitment, not development, since they are often fighting for one and done guys. They also don't need to prepare guys for playing basketball beyond 4 years. This often means stifling players by putting them into limited roles. We also see guys playing roles in college that they can't play in the NBA. Coaches are not necessarily preparing guys for great NBA careers. NBA teams will actually try to help guys find their career niche.