a transponder (which many aircraft have already) squawking 7700 does the same job
Nowhere close! While a transponder squawking 7700 may be useful, it does not give position and may cease to operate or provide useful info at low altitude or on the ground. PLB is for different purposes and serves those purposes far better than a transponder, IMO. We have a PLB for each seat in the helo - about the cost of a tank of fuel.