It's the transfer portal.
A until the past few years, if a college athlete wanted to leave their school, they had to sit out an entire year with their new program, and lose a year of eligibility. In 2018 -- and really gaining momentum in 2021 -- student-athletes could move between programs without sitting a full year.
This (and Name, Image, and Likeness licensing, also called NIL) has resulted in a fundamental shift in the power balance in football programs toward talent and away from coaches/programs. It's not infrequent to see top prospects change schools if they can't move up the depth chart or there's a change in the coaching staff.
For example, it used to be common for Nick Saban to immediately bench and denigrate his recruits, and have them compete for their scholarship every year. Today, if a college player doesn't like their situation, they can enter the transfer portal and find a new program.
You'll often hear teams recommend their players enter the transfer portal also. Coach Deion Sanders famously recommended almost the entire 2022 Colorado team transfer out. The QB for Ohio State was encouraged to leave.
Five of the Ringer's Top seven QB prospects passed through the portal at one time or another: Caleb Williams (OU -> USC), Jayden Daniels (ASU -> LSU), Michael Penix Jr (Indiana -> Washington), Bo Nix (Auburn -> Oregon), and Spencer Rattler (OU -> The Other USC).