Many would blame Activision. And not a few would say that Blizzard sold out. To me they seem to be mostly milking the cash cow, and they’re hardly alone in doing it.
I’m not sure who invented loot boxes, but that’s another (unmentioned) issue entirely.
I guess you could argue league systems are a pvp grind, but that's not quite the same as my original point. Competition naturally includes (relative) ranking.
The phases change based on the expansion, but the core gameplay loop has always been "grind to get better gear, so you have to access to harder content that drops even better gear". There's precious little to do on a character that is max level and has cleared the highest tier of raid (kudos to those who pull it off, they aren't easy).
The monetization thing is largely Activision, or at least the degree of it is. Iirc, Blizzard dipped their toes into that water first by offering pets that you could buy or were rewards for buying Overwatch, etc. Activision did crank that knob up to 11 though.
The original RTS games (Warcraft 1-3, Starcraft 1) did not really contain these systems. But virtually everything else Blizzard has produced (including Starcraft 2) has grinding & collecting in it.
As mentioned, this spread to other genres, infusing Activision's flagship Call of Duty franchise with grinding & leveling. The mobile division, King, does all these things as a matter of course -- it's nearly 100% of the business and design models in mobile gaming.
To be fair, you can't blame Blizzard entirely for spreading these design choices. It was an industry wide shift. But they were one of the shining golden examples of its success, and they sure leaned into themselves.
Assuming gp was referring to these mechanics, there's a lively debate to be had about propensity to spur addictive behaviors.