Because the analogous problem (incoming gear disparity) doesn't exist in PvE. When you start doing PvP, you're matched against people who dramatically out-gear you. When you start doing PvE, you're matched against opponents designed for your gear level (entry level). In other words, PvE has always been balanced in this respect, while PvP has not. Adding bolster to PvE is addressing a problem that doesn't exist while simultaneously carrying with it the problems it has in PvP to an exacerbated extent. PvE is a gear game, and PvP is a skill game. If you take the gear out of PvE, there is absolutely nothing left.
Finally, a pve'r that says something i agree with ..... "PVE requires no skill."