Every GPU cluster design review reaches the same slide: the nodes and the leaf switch are in one rack, nothing is further apart than about three metres, and somebody has to decide what goes in the cable order. The textbook answer is passive Direct Attach Copper, and the textbook answer is usually right - lowest power, lowest cost, one fewer active component per link.
The part that bites is the number people carry around in their head. "DAC does 5 metres" was true at EDR. At NDR it is not, and a rack of 800G links planned on that assumption produces a cable order that does not physically reach.