Because it's unconventional and illogical. Most people looking for gaming headphones tend to have small budgets. For gaming purposes, recommending a $600 amp/dac on the guide just doesn't make sense. If they happen to own such a solution, then chances are they already know to use it. For thise looking for simple, accessible methods on powering headphones for gaming, will more than likely go for something in the $100 range.
I know I have expensive headphones up, but those are more likely to be bought, than an expensive amp/dac.
The other reason it doesn't make sense is that most of the perceived sound quality at the end is a function of the headphone.
Buyers should pick out whatever headphone fits their tastes and then buy the amp that brings the most out of it. Going the other way around is unnecessarily limiting...especially if you're like me and develop a taste for electrostatics at that price range, which rules out the vast majority of the amp market.
Also, I don't really like combo DAC/amps. Want to replace just one? Too bad, you gotta replace both. Besides, if I had that much money to blow on just amplification, I'd grab another Stax SRM-T1, maybe an SRM-313 with a Normal bias jack, depending on what's available on the used market. Those sort of amps never get DACs integrated into them anyway.