Yes, it's mostly power consumption and efficiency, with the puny heatsinks found in most laptops, you can't cool significant amounts of power without throttling

Yes, it's mostly power consumption and efficiency, with the puny heatsinks found in most laptops, you can't cool significant amounts of power without throttling. Having the CPU on the motherboard also allows for a thinner assembly. I don't think having a socketed CPU just for the sake of it would have been a good choice here.