Because most people don't expect a ton of traffic, and generating every page dynamically works until they're surprised to have hundreds of visitors in a short period of time.
So programmers of Blog software think that lots of page visits are some kind of accident? I hope not!
Also, handling lots of requests for small, static content is note exactly a tough problem. (Although it requires a minimum of care.)
To any author who cares about what he writes (publishes), being read by many people is a great chance. However, crappy blog software deliberately destroys all those nice opportunities in advance.
