Well, from what I've seen and read, this is mostly due to the fact that @ruud (am I doing that right?) has put in the time, effort and, most importantly, the money into providing server performance necessary for lemmy.world to survive and handle the load. I wouldn't worry about centralization with Lemmy, simply due to the fact that, should @ruud all of a sudden decide to turn into a first-rate a$$hole (definitely don't see that happening from @ruud's reputation!), the exact same software works on other servers which could supplant lemmy.world should the need arise. Long story short, Lemmy is tyrant-proof, IMO.

On a related note, I would encourage those who appreciate lemmy.world to visit the homepage and contribute when they can, since the associated costs are not insignificant. Who knows what may happen 5-10 years from now; Lemmy could become the dominant platform. I have no problem with that...