Becuause PvP has more variability so you have more reasons to change talents and you have more talents to change (pve talents still work in pvp) and you often change your trinkets as well based on comp.
To use the muti example again; master poisoner is better against comp with healers (so all 3s and most 2s) but elaborate planning has higher throughput (for duel dps comps or wpvp), that's on top of if you want manoeuvrability or mind-numbing poison depending on comp your facing.
That not even counting combat; In pve 99% of the time I would spend cps on envenom, whereas in PvP you often want to use it on kidney
Which is not to say there aren't cookie-cutter specs in pvp, there are just more of them.
That's a silly hypothesis. Cookie-cutter specs have existed since vanilla, people have used guides since vanilla (no matter what ghostcrawler says).
That said. What your cookie-cutter spec should be can change based on; Stat breakpoints, gear breakpoints (rip tier sets),content (raid, m+, pvp) or patch changes.