Because some things don't even show up as a problem until you've encountered them before. I'm not sure about now, but Wordpress used to be nigh-unusable for a

Because some things don't even show up as a problem until you've encountered them before. I'm not sure about now, but Wordpress used to be nigh-unusable for a blog if you got any sort of notable traffic unless you you installed and configured a caching plugin. People that went through that generally did it from then on for any future blog they configured, but that doesn't mean some new blog from some new blogger would have it, or that someone wouldn't forget about it when setting up a new one. The problem is that it's often not a slow degradation, it's an exponential one. Things will be going fine, and then things will look a little slower, and then things will completely fall apart. This is because the few parts that actual lock and cause contention stack, and problems beget more problems. See also virtual memory in servers. Often, that point at which your server needs virtual memory is the point at which it slows everything down more that more and more requests stack up (because virtual memory is slow), needing more memory, and you get a spiral of death in many cases.