That exist, kinda. Android gaming handhelds have mobile SoC and also active cooling. See Ayn and Retroid devices. Their (almost) throttle free is a very nice experience.

Yeah I have an Ayn Thor with a Snapdragon 8 Gen 2, it's great being able to play fairly demanding Windows games on this tiny device.

It's still PoP with the RAM stacked on top in a plastic casing so you need a pretty big fan to cool it since you have to go through insulating plastic, then the RAM to get to the CPU.

With a HPB packaging you can cool the CPU directly and could theoretically get the same amount of cooling power with a much smaller fan that could be integrated into a thing smartphone (à la RedMagic).