My opinion is that MacBook Pros turned into consumer machines somewhere along the way and the overwhelming majority of buyers use them for Facebook, Youtube and Mail.app. I imagine Apple has a vast amount of metrics on this. None of these use cases require better thermals, so Apple engineers the devices accordingly.

I don't recall my 17" Powerbook G4 ever having thermal issues but how many people would buy a ~$5000 base spec MBP today? See Mac Pro.