And perhaps that should change. Guild-based grouping is a far more socially stable environment for the game than the current situation where a very significant number of players does PuGs. There are already communities that have half a dozen different raid groups running using discord to keep people connected. Making guild achievements impactful and bring back guild perks without the ridiculous leveling requirements would give people a good reason to join a guild/community. And letting people join multiple guilds would remove the opportunity cost. I have had people who were raiding with my guild but wanted to stay in their own social guilds outside raids because of RP/friendships and especially back in Cata with the legendaries there was a lot of pressure for them to leave their guilds if they wanted to be eligible for the legendary.
You could even make guild challenges matter again (do they still exist?). That could make people want to run group activities with their guild instead of a PuG.