Because you'd never put your credit card into an HTTP web site, but you would on HTTPS. Your argument about MITM being uncommon is moot because it's not imposs

Because you'd never put your credit card into an HTTP web site, but you would on HTTPS. Your argument about MITM being uncommon is moot because it's not impossible and is only rare because the current system is the way that it is. Changing the system would change the attackers' methods.