I've never been a grad student, so this is pure supposition, but...

I suspect that if you went into a PhD program and tried to hand in a thesis six months later, the response that you'd get from on high is "Ha ha, very funny. Come back in three years", and that this response would happen whether or not you produced something that's actually good enough to be a proper thesis. Profs know how long a doctorate is "supposed to" take, and doing it in a tenth of that time will set off alarm bells for them.