Sometimes you can get laptops with decent CPUs, but they only have onboard graphics
Sometimes you buy a system that physically can’t house the GPU you want, but the board/PSU is proprietary, so you can’t just change the tower/case.
Sometimes there are GPU heavy tasks that don’t really rely on CPU that much, and you are using a laptop.
But the vast majority of the time, sticking a new card in your desktop is the way to go.