Cali explained a lot already, but essentially it comes down to the fact that if people are looking for a new guild they will almost always go for a guild that is higher in level. Why, because those guilds have more benefits then a new guild. Like Cali said, it's the path of least resistance. We can argue that you don't want people in your guild that act like that, but that's a different discussion. Back in Cataclysm I lost more people in my guild then I could recruit back. Cataclysm made a lot of people quit and it generated into a snowball effect, so even more people left. The only reason I kept playing longer was due to the fact I had a responsibility towards my members. So my guild slowly died and eventually I had to bury it. It's sad, but it happens. So in short, people are less likely to move guilds if one is lower or less likely to join low level guilds because of the existence of perks that are too good to pass on imho.