An interesting study has found that, contrary to popular opinion, engineering programs don't have particularly attrition rates. (By attrition, in this case I mean switching to another major rather than leaving school altogether.) But colleges still graduate fewer engineers than they start with, because vanishingly few students switch into engineering from another major.
The comments to the article I link to suggest several reasons this is the case, some more plausible than others. One suggestion is that engineers come in with a stronger sense of what they want to do. It's possible, although I know of no data to evaluate this with. Another is that engineering culture is full of loud-mouth, drunken, conformist asshats. Again, I don't think there is data readily available on this point, although in my undergraduate experience engineers tended to be nerdy and while often conservative, hardly louder or drunker than anyone else - in fact they were more often teetotalers or antisocial.
The obvious reason, and one we can test with data, is that degree requirements are much more specific in engineering programs than in most other fields, and switching to engineering after even a semester would likely delay graduation. The next step for an alert graduate student would be to start examining course catalogs, then to perhaps move on to gathering data on what courses entering students actually take.