What you and other people don't understand...
If you create a new damage table for PvP (or just shift it to another stat) then you just create additional testing and balancing issues. Now the developers have to check twice the results every time they make a change. Now its twice as long to roll anything out.
It also creates discrepancies with player expectations. "Hey! This ability usually hits for 2000 in PvE, why the heck does it only hit for 500 in PvP?"
There are many, many reasons to keep the game consistent. Not to mention that PvP gear is often mixed with some PvE gear based on how the player wants to build their stats. This solution, while neat on paper, would be a mess in real life.
you assume that the devs are even doing this properly in the first place, which they clearly are not..........
keeping the games consistent < balancing the both the pve and pvp aspects of the game. the OP has a really good idea, tho i dont think it would work.
iirc, expertise is added outside the initial attack damage calculation. so the devs would have to re-write most of the code for abilities. which, imo, is not that complicated and has already been done once before in this game. this solution would be worth exploring imo