The damage being relative to the cost of the rifle is perfectly sensible. If all rifles could one tap everyone would just buy the famas / galil because they're cheap and the whole economy system would be irrelevant. Or even worse, if SMGs could one tap everyone would run the UMP. Buy every round boys!
Where's the advantage if every gun would have the same power? I'm sure you're baiting at this point but why would anyone pay 2700 for an AK or 3100 for an M4 if a UMP did the same job for 1200? This literally happened when Valve accidentally overbuffed the UMP in the past as they increased SMG armor penetration. People would be buying every round and often favouring a UMP + full utility over a rifle with slightly less utility as the UMP was so strong it could rival any rifle at close to mid range.
Especially since it was before the damage dropoff increase so you could easily killed people at med-long range with two shots as well. This submachine guns meta was terrible (not as much as double scopes meta though).