[Sohcahtoa82]

One word:

Heat.

Smartphones are decently powerful, but they typically only use the CPU in short bursts. This has allowed them to get away with being extremely skimpy on the cooling, which is good because a phone that needed a heat sink and fan would be incredibly bulky.

If you ran something with sustained CPU usage like F@H, within a minute it would be considerably warm and probably have crazy amounts of thermal throttling.