Because if it actually is worth 2 attacks it would completely outshine Warrior in every way. Base AD ranges from 80-110 at level 18 (generally; there are some outliers). If every 2nd attack your attack did 2x damage Devourer would add 80-110 damage compared to Warrior's 40 flat AD (and remember, Devourer has 30 on hit damage, stacking up to 60). The unique sated alone would be better than Warrior in 100% of the cases.
Additionally, that opens up much more frustrating abuse cases; not many people enjoy Kayle E'ing through a team, but I can guarantee even less people would enjoy a Rengar crit Q doing 475% AD in one hit (crit on empowered Q, assuming your "2 attacks in one" would not be capable of critting and would use total AD). Overall, it's fine that the AS focused jungle enchantment benefits autoattack focused junglers more, as that is the enchantment's primary purpose. Currently the enchantment is great with on-hit effect champions and meh on others; changes like the one suggested here would either make it decent on on-hit effect users and better than Warrior on anyone else, or worse than Warrior for everyone except a very select few (ones like Warwick maybe would still use a weaker, on-hit effect Warrior).