Ruby has a lot of web frameworks too. All your experience means is that you don't pay much attention to Ruby (which is fine, I'm only vaguely aware of the 3 you listed for Python)
The Merb/Rails merge reveals some reasons why there aren't a lot of widely used Ruby frameworks, and I'd say it's mostly a large scale manifestation of DRY, combined with the maturity and wealth of features in the Rails codebase and the fact that most of the changes devs would like to see in Rails are already going into it.