Forgetting Darktide, the Zek servers, and Ultima. All of which had great PvP.
Well it depends. A lot of time is spent creating art assets for unique dungeons, i.e. reef colored walls in Blackwater Depths. If you only populate your world with one model, "a kobold" and just rename every mob some derivative of that: "kobold avenger", "kobold sniper", "kobold assmaster" then obviously you'll be able to perform things much quicker. However, if you want a bunch of unique mobs then you need different models for each one and a lot of time and resources are spent on that.
That's why I'm an advocate for a company using a Diablo/Ultima-esque 2d over-the-shoulder engine to speed up content and make features so much easier to implement. There's a reason Ultima Online had mounted combat and no other MMO since has. It's because they were able to do the animations for it without much difficulty and in 3d games, it's just too much effort.
Content usually requires a lot of time -IF-it's done well and unique.P