is definitely a downside of possibly having to move to a worse school/program or finding no landing spot at all.

For a good player, the team they left would usually welcome them back if they decide they want to come back. Or perhaps the team moves on, but they would still find a good landing spot elsewhere. So they get the extra tasks of having to move to a new location. Which probably interferes with their academic pursuits. but overall, little downside for a good player to enter the portal every year.