That would be in the middle of the semester. It's where it is in the first place so they can be part of spring ball at their new school. I don't think it benefits anyone to have players hanging around spring ball that are already out the door at their old school while not being able to participate at their new one.
If schools were more willing to accept mid-term transfers, then fantastic. But from my limited knowledge of how that works, it doesn't seem like a realistic possibility. Otherwise the portal would already be there.
It's also more ideal if high school signing periods and portal windows were in alignment. If a high school standout QB signs at a place based on opportunity, but then a transfer portal QB lands there a couple months later, then you're just creating another batch of transfers. If those windows are aligned, everyone - high schoolers, transferrers, coaches - can make better, more informed decisions.