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It's simply length. Ethernet is expected to work on 50-100m runs, while USB4 specifies maximum cable lengths of 2m even for just 5gbps (at least for passive cables). 80gbps is 0.8m

The longer Thunderbolt (which is actually just USB4) cables internally use fiber optics for data transmission, with converters to copper in each connector. Even the medium-distance (3 meter) ones have signal quality boosters in each connector matched to the kind of signal degradation that kind of cable will experience.

Completely passive TB4/TB5 cables max out at about 80 centimeters.

Besides what retatop and crote said - USB4 uses either binary or PAM-3, 10GBASE-T uses PAM-16. Higher modulation means lower frequency bandwidth (so longer and/or crappier cable), but also more current-hungry line driver and more current required to keep noise manageable.