In recent times WOW has made some changes to the way raid lockouts work. It used to be that each instance of a raid had a unique ID and once you joined that instance you ended up saved to it. This was a powerful force in raiding because it prevented people from playing through a raid multiple times for loot, practice, or just entertainment. It was a big reason that guilds were so powerful and guild loyalty was such a force. If your guild raids SSC then you cannot go to SSC with another group because you will end up locked out of the guild raid, which means if your guild needs you later in the week they are out of luck. Raiding a zone with outside people was treated as something of a betrayal, practically the same thing as quitting raiding with your guild.
These days the mechanic is both different and better. Each player can only loot a given boss once per week but they can fight wherever and whenever they like. I can go through a raid with some friends on a lark and then when my guild raid happens I can go through again, though I won't be eligible to loot the same boss the second time around. The effects of this change have become obvious as time goes by. I often see people running mostly cleared raids in PUGs in the hope that they can get another boss down. Back in the day it was a big deal to sell guild raid IDs and guilds would end up purchasing them from one another occasionally to do something specific. Selling your guild's ID to another group was a betrayal of the highest order! Now that those IDs are a thing of the past you can join up with a PUG to try to kill a particular boss and the worst case is that you kill it again with your guild and only get loot once. Not so bad!
It seems to have changed the way guild loyalty works. People constantly raid outside their guild with alternate groups because there is no penalty to doing so aside from the time investment. It is almost like there is a palpable decline in tribalism, as though the attachment guilds and members feel to each other is lessened. I can see how some people might view that as a penalty but I think it is a good thing. The sense of betrayal and bitterness that showed up when someone left a guild was terrible to behold and now there is less need for that. You can more easily raid with your friends and are less constrained by the artificial construct of a single guild membership.
Flexible raid sizes surely contribute to this too. When you must bring seven healers to a zone and one of them quits WOW or raids with someone else you are all in a bad way. Unless you are raiding mythic difficulty though you can just run with less people and be fine. If you find a new recruit who seems promising you don't have to bench them until you have space... just bring them along.
Guilds these days feel more like a way of creating social spaces rather than organisms savagely competing for scarce resources, jealously guarding their hoards. My sense is that it is a lot more comfortable to run things this way, to have players feel more free to go where they will and fight what they want instead of being locked into groups that feel extremely possessive of them. The sense of loyalty to a guild feels like it is lessened, but so is the sense of obligation. It is a tradeoff that is very much worth making, I think.