Thunderbolt 4 is a fast port but a slow bus. It's effectively 4 lanes of PCIe 3.0. High-end consumer SSDs typically expect PCIe 4.0 x4, or twice as much bandwidth. Thunderbolt 5 is supposed to provide that once it's finally released, but by then PCIe 5.0 SSDs will also be out, doubling the bandwidth requirements again.