IIRC the specification of Thunderbolt from Intel (which was inherited by USB 4) limits the Ethernet emulation mode to 10 Gb/s.
Why did they set a limit so low is not known, but the supposition made by another poster that this is a market segmentation feature may be right, because such policies have always been typical for Intel.
This post is getting almost 12 Gbps and someone mentioned getting 16 Gbps on Mac so I don't think there's any intentional limit. Thunderbolt is really 22 Gbps anyway and networking over Thunderbolt is just inefficient.
Why did they set a limit so low is not known, but the supposition made by another poster that this is a market segmentation feature may be right, because such policies have always been typical for Intel.