Because no one has yet bothered to develop an IP other than Intel. Much of the IP especially for PHY is licensed from other companies especially in the case of

Because no one has yet bothered to develop an IP other than Intel. Much of the IP especially for PHY is licensed from other companies especially in the case of AMD. AMD only recently has introduced CPUs that can be put into premium laptops and still they don't have a CPU with more than 4 cores and an integrated GPU. With Intel integrating TB into the CPU directly it also would mean that no one is currently making discrete TB controllers Titan Ridge is the last controller you can buy as a discrete unit and these can technically run on AMD systems but the cost doesn't make it sensible. With USB4 there will be TB3 compatible IP from multiple 3rd parties which AMD and OEMs could license to integrate it into the SoC directly or into the system itself via PCIe.