For Lightning, yeah, there's basically no reason except for the 'Made For iPhone' program, and it isn't even effective at that since Chinese cable makers just clone the keys of the official stuff.
40Gbit DAC is even more annoying because unless the cable is a bit more active, it's essentially 4 10gbit DAC in one sheath.
Second, yes, some does require active cables with DAC. which does not stand for "cable" like the other comment says. DAC is direct attach copper (as opposed to optical). Without an active cable you're limited to 5 meters, although you can risk it and get a 7 meter passive cable. usually, 7m is already active, and 10m+ is guaranteed to be active. The reason you can even do 10G over copper is because it's cat6. Have you seen that cable? It's twinax - more like the cable from your TV antenna. That's not viable for a phone.
What you are talking about is 10G base-T. That does not need active cabling. but again, you can't compare that to a tiny thin lightning cable.