Limited RAM, even a 4090 only has 24GB and slow transfers to/from VRAM. The GPU can only operate on data in VRAM, so anything you need it to work on you need to copy over the relatively slow PCIe bus to the GPU. Then once it’s done you need to copy the results back over the PCIe bus to system RAM for the CPU to be able to access it. This considerably slows down GPGPU tasks.