100% this

The PCI-Express bus is actually rather slow. Only ~63 GB/s, even with PCIe 5 x16!

PCIe is simply not a bottleneck for gaming. All the textures and models are loaded into the GPU once, when the game loads, then re-used from VRAM for every frame. Otherwise, a scene with a lowly 2 GB of assets would cap out at only ~30 fps.

Which is funny to think about historically. I remember when AGP first came out, and it was advertised as making it so GPUs wouldn't need tons of memory, only enough for the frame buffers, and that they would stream texture data across AGP. Well, the demands for bandwidth couldn't keep up. And now, even if the port itself was fast enough, the system RAM wouldn't be. DDR5-6400 running in dual-channel mode is only ~102 GB/s. On the flip side the RTX 5050, a current-gen budget card, has over 3x that at 320 GB/s, and on the top end, the RTX 5090 is 1.8 TB/s.