First, not all highly recruited players coming out of high school are suited to play at the FBS level. I say this because at the high school level, in many cases, they were absolutely the best on their team. When they arrive at a major football program they then realize that every teammate is an outstanding player and regardless of talent level there are no guarantees of playing time. This is especially true at QB since it is the one position that is not a situational position.
Every other position requires rotation of personnel due to game participation wear/tear and circumstances, but not at QB. These non-QB players have to accept that they may or not play their first year, but if they do it may be on ST or coming in for a particular situation. They have to learn to be a role player since the grind of a 12-15 game season requires having adequate backups. During the season as a backup their roles expand and they gain valuable experience. Some players do not see this, get frustrated with the lack of playing time, and wind up heading to the Transfer Portal. Not every recruit can adapt from being All American and All State to being a role player or being redshirted with no chance of playing. It certainly has to be a hard lesson for some of these kids to learn and possibly the reason for so many transfers.