Maybe because some have tried it and discovered they didn't really retain what they read. This might easily be a personal thing, not an indictment of speed reading at all, only that some people can't absorb printed matter quickly.

In other words, the same training with the same goal, but completely different outcomes.

Another issue is content. Some of what I read requires great concentration and repeated scanning of complex passages. Other content can be read quickly simply because it doesn't require much from the reader. So one might say that not all reading is created equal.

Example: how quickly can one absorb "It was a dark and stormy night"? I would guess pretty quickly. On the other hand, "Time flies like an arrow, but fruit flies like a banana" might take a little longer. :)