Because there would always be people to complain about imbalance even when there wasn’t any ?
Ex : “hi im pally and cant do a sh!t vs mages in duel cuz slow and sheep and spell steal wtf this game srsly trash balance fix it blizz ffs ty”, etc…
Back to the matter at hand, namely PvE, I first took my break from WoW when Cata hit because it kind of killed it to me, but I do remember some people praising class balance when WOTLK hit because it solved many Vanilla/TBC issues. Basically any class back then had a spot in endgame, and was even kind of mandatory. Though druids had it harder in ICC because of the dodge debuff, there was no unviable tank even though there was a best one. Healers were all equally viable, even disci, and shamies were top only because of their braindead gameplay - they still didn’t perform any better. Ferals were as good as rogues, shadow as good as warlocks, balance as good as ele and enhanc was a mix of pure destruction and fun. With the exception of frost, arms and sub being exclusively pvp, all the rest of the specs were performing just fine and, on top of that, actually had their use in raids.
PvP ? There was no such thing as “unviable” spec, let alone unviable “class”. Yes half of arena combos were warms/hpal, but you could find just anything else. Back then I had cap’d 2k on my ele and my mate was… assassination rogue. We’d melt the hell out of warms/hpals.
Know why shamies suck since Legion ? Because they scrapped them off their totems. They were nothing “weak” when BfA hit, people just complained about their lack of utility, and it was more than justified. So in order to address the issue Blizz found no other option but to make them ridiculously powerful. Guess what effect it had on other classes… Behold balance at its finest.
And now the game got dumbed down so much it’s becoming harder and harder to balance it, ironically.
Same goes for MoP, which I didn’t play that much, but I do remember almost every class being as fun and viable as any other, and you could basically be anything you’d never feel boring, or clunky, or slow, or useless, or anything you’re prone to feel nowadays if you play any melee other than dh, rogue or war (exaggerating of course).
Yes, all specs are playable in +10, but the difference in each and every one’s performance is far from being absent. When I tank on my overgeared dh, I feel so sorry for the healer. When I tank on my undergeared dk, I feel sorry for my dh. When I queue as dps on my dk, I hold my breath until the leader puts their key because I’m afraid they might find out I’m frost and actually kick me. And honestly, I wouldn’t blame them. Having played frost dk in Legion makes me wonder why I’m still playing it now. And better I don’t get started on arcane… hell even wotlk’s braindead rotation was more efficient.