As I said, because right now it doesn't actually do anything. So why are we making it harder for student athletes for no gain. It's a charade.
If you want to make it stricter, then you're actually harming students who transfer, not just inconveniencing. There are many student athletes who are forced to transfer for very legitimate reasons. So then you're really valuing the game over the quality of life of people. I find that morally wrong, but we may disagree. You could add waivers and exceptions. Depending on how it's executed, I could even be okay with that system, but as we've seen finding the balance between tough to deceive/exploit and working as intended to prevent unjust injury is extremely difficult.
At some point, you have to try and protect the competitive balance of the game. A lot rides on that, including the public's continued support (read money). Where that line exactly is, I'm not saying.
Lots of jobs are on the line as well, people's livelihoods affected by (what can be) the whims of teenagers. And I'm not talking about the wealthiest head coach, but lots of lower level people involved.
I don't think it's harming students egregiously, if at all (it doesn't scholastically in the least). In fact, it's helping them on a number of fronts. You can get an extra year of schooling if you wish, to simply focus on your grades, bank classes to spread the workload, or use towards a graduate degree. And from an athletic perspective, many transfers have commented on the benefits of the developmental year off.
If you transfer, it essentially gives you another $50,000 and more opportunity. Really depends on how you look at it.
In the end though, it depends how the rule will be exploited. And make no mistake it will be. There are obvious places where exceptions comes into play.