The Intel MacBooks had bad thermals because Intel had promised more efficient processors and Apple had designed the new cases around that. Then Intel flubbed their die shrinks for several years in a row and Apple was stuck with the hotter chips from the larger process node.
Since they were already on the road to switching to Apple Silicon, it may have not been worthwhile to redesign the cases for the Intel machines to compensate.
Intel flubbed the die shrinks several years in a row, but from 2015 it was pretty obvious that whatever apple was designing for wasn't enough. There were better thermal solutions in thinner, cheaper laptops. The fan didn't even draw air over the CPU in the last intel MacBook air. I opened one and really tried to figure out if there was some 4d chess going on, but no. The fan did nothing. It just started spinning fast making noise for no reason. I even did the old incense test. Next to nothing was being pulled over the CPU heatsink.
It was spectacularly bad by design. This would not have been sufficient even if intel had delivered.
The pro models were better but still awful. It wasn't a matter of fitting things in. It was by design.
I have apple products. I like them. But even being charitable I can't come to any other conclusion that they just didn't care to deliver a good product.
Since they were already on the road to switching to Apple Silicon, it may have not been worthwhile to redesign the cases for the Intel machines to compensate.