Boss mechanics don't change. They do between modes but as soon as you start whichever mode you desire, be it vet or HM you know exactly what attack the boss does and when.
The mechanics in their basic form don't change, that's right. But the sum of all mechanics, and the resulting possible combinations of situations and events require much more flexibility and adaptability than some PVPers are apparently willing to give PVE credit for.
If that wasn't the case, like if PVP really required so much more skill like some people here claim, any moderately good PVP group should be able to faceroll through say Asylum Hardmode, right?
Somebody mentioned earlier and i agree with them. A top pvp player can adapt to be a top pve player. Its not the same the other way around. Obviously some can but imo, if you are very good at pvp, you are good enough to do all pve content. If enough practice is put in. Obviously mechanics need to be learnt.