[somanyphotons]

What allows DACs to avoid the power/thermal issues that twisted pair has?

(My naive view is that they're both 'just copper'?)

[kijiki]

DACs are usually twin-ax, which is just 2 coax cables bundled. The shielding matters a lot, compared to unshielded twisted pairs.

Faster parallel DACs require more pairs of coax, and thus are thicker and more expensive.

[p_l]

Another reason is that they are shorter range, and the better shielding also means that interference effects are smaller.

In comparison, twisted pair sending 10Gbit over 8P8C cable (popular "RJ-45") requires complex modulation schemes to provide solid signal over any meaningful distance, in a much less shielded cable, and with need to support longer distances.