Because Blizzard are not infallible.
The intended design was that PvE gear would be best for PvE, and PvP gear would be best for PvP.
The former half of that sentence is successful, the latter is not.
Let’s look at why.
Blizzard designed some Azerite Traits specifically for PvP that would be obtained through PvP. These were meant to be the best for PvP. But it turned out they were not. Blizzard made some balance changes to Azerite Traits in PvP to alleviate the problem, but alas, it has persisted.
The PvP trinkets were supposed to be the best for PvP, but again it turned out that they weren’t. Blizzard tried to alleviate the problem by introducing new and more powerful PvP trinkets to the game, but alas, the problem persists.
And now that Blizzard are introducing more unique item designs for PvE, the problem seems to spill over to even more slots – weapon, pants, gloves, boots, etc…
Alas, Blizzard have simply failed to accomplish the design they aimed for. That is why PvPers feel forced to do PvE.
The underlying problem is that PvEers request more interesting, unique and powerful items from Blizzard – and Blizzard gives it to them.
On the other hand, then the PvP community requests more simplistic, common, and less impactful items from Blizzard – and Blizzard gives it to them.
That naturally results in PvE gear being superior to PvP gear, and when Blizzard then tries to balance the scales by introducing more powerful PvP gear (Maledict & Safeguard), then PvPers complain, because that’s not what they want.
And there’s no easy fix for that.