Because the spec and implementations came along well before we started putting HTTPS on everything, or even most/many things. Given the number of HTTPS->HTTPS l

Because the spec and implementations came along well before we started putting HTTPS on everything, or even most/many things. Given the number of HTTPS->HTTPS links you would have run across in regular practice the spec and your proposal were probably more or less identical in practice. As for why it's still that way... I'm sure no one has bothered to really think about it since.