Looking at the amortization schedule for the $400k/5% loan I reference earlier (I have it using the mortgage calculator from which I derived these numbers, I can post a screenshot if you want to see it), if someone were to sell their house after exactly 4 years/48 payments of $2,147.29 each month (total payments of $103,069.92), they would only have $25,480.00 in principal built up from that original $400k they borrowed and the bank would have made $77,589.92 on interest payments; which means the bank would receive 75.279% of the payments made just for loaning out money for 4 years. Based on prorating the interest across the life of the loan, the bank would still receive 48.255% of the payments made and the borrower 51.745%, meaning the bank would receive $49,736.39 and the borrower would have equity/principal of $53,333.53. For a whole lot of folks, they'd make the argument that receiving almost $50k in 4 years for loaning $400k (plus fees) is more than enough and that the bank is doing just fine with that amount, especially as the bank would now have their $400k back plus another $50k more to make other loans that they could use to make additional money.
As I've already given caveats about, I understand that such a regulation would greatly change bank behavior regarding loans because it would change the profitability calculation. But I also said from that for folks who don't really get into the details of financial instruments, that I can see why the idea of prorating interest would seem like a great idea.