50% of D1 players transfer to play some place else (saw it in BA a few years ago). I’m guessing D3 players who don’t pan out quit playing and focus on academics or partying.
At a D1 there are five to seven top players who are a lock to play. At the other end are eight non scholarship who typically need a break just to get a shot. In the middle are a lot of players of reasonably equal ability. The mentally tough players who stay mentally prepared for when they get their shot succeed.
In any given season only 18-20 players on a D1 roster receive adequate playing time. The rest are on the bench thinking about transferring if their shot doesn’t come soon. The most likely to stay another year are freshmen who see their opportunity coming next year.