They do have their afterburner card, and the Mac Pro already uses a modified version of PCIe. The key difference is the M1 macs are laptops and all in ones, and the Mac Pro is a modular system. It’s a different beast. The MacBook Pro has to be power efficient as well as fast, where a desktop computer like the Mac Pro does not have that limitation. The current Mac Pro already uses infinity fabric bridges to link the graphics cards. Apple could easily design an accelerator card that has everything you need in one card. A GPGPU isn’t hard to design. You just have to understand form factors.