Yes they made it thicker, but as you see, without much gain. They just made it look thicker, with an innuendo that this is for better thermals, while in reality it isn't.
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.
