Different hardware type. Gaming laptops have dedicated graphics cards which generate heat from an additional source, and they have to drive 1080p/1440p/4k content, whereas the steam deck is a 1280x800 screen, which is absolutely perfect for an AMD integrated GPU with reduced thermal management.
The steam deck is a single spec tightly tuned machine and software package not unlike a game console, whereas a gaming laptop is an all purpose machine with hardware all over the spectrum that you can buy what you want/need.