The problem is that not every school plays by the same rules, at least not in Hillsborough County. In theory, any family can move into a particular school's district and then be able to attend that school. Also, that same family can stay right where they are and their kid still has the right/ability to attend any other school in the district/county, with one major caveat: the school cannot be at or above capacity. So, a school in a less densely populated part of the County with a mediocre academic reputation/ranking, that is not at capacity, can accommodate an unlimited number of transferring students. Whereas a school in a more populated area with a strong academic reputation/ranking, that has been over capacity for years, can only accommodate a transfer if that kid's family moves into the district. And, of course, private schools can do whatever the heck they want to and, thus, can accommodate any kid they want to whenever they want to regardless of where the family lives.
Free agency doesn't work unless all schools are playing on the same, level playing field and that simply isn't the case. My guess is that Lakeland, for example, would see far fewer transfers if kids who want to transfer into Lakeland actually had to move into the Lakeland district. That's not a knock on Lakeland; it's just a reminder that not all public schools are in the same 'boat.' That assumption is based on some quick research that indicates that Lakeland's student population is well below capacity. If I'm wrong on that, please correct me with the right info.