its because macs' tb4 ports dont support usb 3.2 2x2 speeds of 20 Gbps. in fact very few pcs do. it came pretty late into the usb lifecycle and manufacturers already started adopting USB4 instead. you can only get 10 Gbps with usb 3.2 gen 2

You can see here the example as given by kingston for their XS2000 external ssd which supports usb 3.2 2x2



only way to get over 10 Gbps r/w speed is by using a gen 4 nvme ssd in a USB4/tb3/tb4 enclosure