PCIe 4.0 x16 (16 lanes) has a bandwidth of 32GB/s, which is 256Gbps.

So external GPUs are always going to be inferior. Also, an external case incurs a lot of extra unavoidable costs. Essentially you have a mini-PC case, a TB to PCI-e bridge, a PCI-e interface and mount and possibly cooling as well.

It's a nice idea and has some applications but on-chip GPUs as part of a fully-integrated SoC just makes way more sense for almost everybody.