Figured the "social" was implied, but thanks for clarifying.
Generally, there's a MASSIVE amount of "PvE" content beyond raiding that can be social and require some level of teamwork/interaction. Open world dungeons that can't be solo'd, group-oriented puzzles that require multiple people to coordinate for shared goals, tons of smaller scale content with players coordinating in small groups across multiple locations etc. It's not all just "group for a raid/dungeon."
Same goes for PvP, plus all the different flavors of PvPvE content that can be mashed together.
Beyond just PvE/P content there's all kinds of other ways to promote social play. Crafting that requires multiple crafters working together towards shared goals with systems designed to connect them and allow for easy coordination is one example. WoW is even taking a step in that direction with work orders: Which are a more social way of interacting with crafting and the economy than simply having an AH you list stuff on.
GW2, especially later on, has a lot of these "increased social" elements even if folks largely don't chat and simply work together with others under the leadership of a few folks. Lots of big zone-wide meta events requiring teams to coordinate, open world bosses with mechanics requiring players organically work out things like killing multiple bosses within seconds of each other lest they revive etc. I think folks are taking "social" way too literally in what was, again, an intentionally vague tweet : P
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"The world" as in Runetera, is built and evolving. That's all he was talking about. I skipped through most of the video and it was overwhelmingly focused on the lore that Riot has built and telling players about the different races, factions, locations, sub-locations, historical events and their significance etc. All stuff that would normally need to be built from scratch for a brand-new MMO, but all exists to build off of for the Riot MMO.
Again, they just started hiring for the "unannounced" MMO (because it's not officially announced) a few years ago. The game could very well be in full production by now (it's been a few years, even with covid), or that could be starting soonish. Unless you're 35+ it's unlikely you'll be in your 40's when it releases : P
As for why...probably a me thing. I find a lot of YT creators annoying, but the guy definitely knows his stuff when it comes to LoL universe lore.