A 12v drill can have "exactly" the same power a 24v drill. That is not a problem. To have them last the same amount of time in drilling, the 12v drill will need a bigger battery in A/hrs.
This adds cost as battery cells are expensive and are the main cost of a drill.
So, as batteries are expensive compared to knocking out motors en-mass, they go for larger voltage motors and then the batteries do not have to be as big (or as heavy, which is major point). They are balancing one against the other to keeps costs down. That is using the same type of battery of course, say Ni-Cads.
Current marketing means that generally the larger the voltage the greater the power - so the punter then can gauge the products on the market. All understood, all de-facto standard.
Now, a 12v drill can be the same power as a 24v drill, no arguments about that. Battery types are changing with Lith-Ion being introduced. That means that a 12v drill can be the same power as 24v drill, but put a Lith-Ion battery in it and matters change as these can be so much smaller in physical size and hold so much more energy. So a 12v Lith-Ion drill can match a 24v Ni-Cad drill in power and battery life and be at least the same weight, or even lighter. BUT!!! That is comparing apples with oranges (Lith-ion and Ni-Cads).
Using the same battery type we are back to stage one. Using 12v and 24v Lith-Ion with the same power still means the 12v drill will need a larger battery in A/hr to last the same amount of running time. Although all drills using Lith-Ion will be smaller & lighter than Ni-Cads.