This is my experience as well. I don't think Google Maps knows the difference between a road bike and a mountain bike. Around here, they definitely include single track mountain trails as bike routes. You have to be real careful with it. There is a disclaimer on the version of Google I use that says bike directions are in beta, and for good reason.

Many states publish bike maps. Most at least will show shoulder widths and traffic volumes of state highways. But they might completely neglect some excellent county roads. You pretty much have to get lucky or find someone with local knowledge to find those.