I was referring to driver-side optimizations, though, not DLSS. That's something that can only be added by Bethesda themselves (mods excluded).
One can surmise that they only added FSR 2.0 due to its being an AMD partnership title, but I suspect there's a lot more to it than that. It's not clear at what exact point in the development cycle of Starfield that versions for other platforms were dropped (if any were ever being done), but Microsoft finally acquired Bethesda in March 2021 -- that's a pretty short amount of time after the Xbox Series X/S platforms were officially launched.
It's also around the same time that AMD launched FSR 2.0 and it seems to me that the decision to focus solely on an upscaling system that could be employed on the Xbox and every PC that could run the game was made around that period, too (perhaps no more than 12 months later).
The AMD-Bethesda partnership wasn't announced until just a few months ago but such agreements take a while to put together and set in stone, by which time I suspect Bethesda had already finalized the engine and wasn't going to alter it after that point. It's possible that AMD genuinely didn't know if the devs were going to utilize other techniques (its marketing division isn't the best out there), hence the silence when initially questioned about it.
Modders have shown how easy it is to add DLSS and Frame Generation to the game, so the lack of support for them (and XeSS) shows that the 'blame' can't really be attributed to AMD, Nvidia, et al -- this is a decision by Bethesda (and possibly Microsoft).
