Historically, this is one of the reasons Apple went with Intel CPUs to begin with. The PowerPC G5 was a nice processor but never ended up with a thermal envelope acceptable for a laptop. So from 2003 to 2006, you could buy a Mac G5 desktop, but if you wanted a laptop, it was a G4. 2006 was the beginning of the transition to Intel, who made better processors that Apple could put in laptops.
It's not the only reason Apple switched to x86, but it perhaps the most commonly cited factor.
I complain about how hot the i9 gets... but then I remember the period where Apple was transitioning from G4/G5 to Intel Core 2 Duo chips... in both cases they were _searing_ hot in laptops, and Apple's always fought the battle to keep their laptops thin while sometimes sacrificing anything touching the skin of the laptop (unlike most PC vendors, who are happier adding 10+mm of height to fit in more heat sinks and fans!).
It's not the only reason Apple switched to x86, but it perhaps the most commonly cited factor.