One theory is that the high voltage and current in a laptop’s battery generate heat.
Secondly, there may be something about laptops themselves or how they operate–such as the long-term heating up of components like CPUs and GPUs–that also cause this heat production.
The third major factor is the natural heating on them from warmer air due to laptops being things we typically use on our laps while sitting outside or on beds, couches, etc. This type of “external heat” doesn’t usually go away because can’t get away from it since we only have clothes on which block some of it while wearing a laptop makes you feel hotter during winters too due to reduced airflow so effectively what’s happening here is one side of yours.