The answer to that is yes! Obviously if a movie has enough popularity to spawn 4 sequels there is a market for it.
I do need to update the original equation of the question from cars + explosions + bad acting = seriously lame! to
cars + explosions + bad acting = seriously lame+millions of dollars in profits
The F&F movies are Hollywood garbage personified! And Americans love it :)
I'm already waiting on line for the 6th movie "The Fastest and The Furious-est"
I think they are a "Guilty Pleasure" franchise. You don't need a story line or plot. You don't need to see the first 4 movies to enjoy this one. You know exactly what you get, and its in the title. Something Fast and Something Furious.
The franchise never promises anything other than that.