Not only WoW, but most old MMOs were built around being social experiences. The really old ones (Everquest, most notably) were basically chat rooms with games attached. The gameplay was very slow, and you relied heavily on other players to progress, so you spent a lot of time just chatting with people, either in zone chat or in groups or in guilds. Over time, you started to recognize the same names showing up in the same places, or as you progressed, the same players would be progressing at the same pace so you'd keep seeing them as you moved from zone to zone.

It was also a lot easier to build friendships for otherwise socially awkward people. You had an immediate common interest and common goal (advancing in the game), so you had common ground to talk about, and a common activity to enjoy together, but during the downtime, conversation would often shift to other things - where you lived, how old you were, what your hobbies were... so you'd get to know people 'outside the game', too.

Nowadays, WoW and other MMOs are much more fast-paced, and much more solo play oriented. There's still group-required content, but it's very action-heavy; you don't have a lot of time that you're just sitting around chatting, and groups are much more short-term things. 15 or 20 minutes, whereas once upon a time, it was 3+ hours as standard.

I met my oldest friend in an MMO about 24 or 25 years ago... we accompanied each other to a few different games over the years, and now we aren't playing anything together, but we still talk. I flew across the country to attend his wedding a couple years ago. Similarly, I met my wife in WoW. Our first "date" was killing bugs in Silithus together. We've been together for about 18 years.

Old (as in, early-late 2000s) MMOs generated a lot of friendships; this isn't at all an uncommon story to hear from people who played them at that time.