Popularity and power seem to do that to people quite often, don't say? Which is said, especially in PG's case, because I like his older essays a lot. The now ones, well, not so much.
As you get richer and more famous, it becomes harder and harder to find an editor/friend you will trust to tell you you're being stupid. You can use the internet at large (that's always full of people to tell you you're stupid) but it can be quite hard to filter.