There are detergents in gasoline.
Unfortunately, there are just enough detergents for the average person who fills up their fuel injected car with fresh gas every two weeks.
For those of us who have boats, toys, OPE, etc.... which don't always run on fresh gas or often enough to stay clean... well, when you clean one or two carbs a year... you learn the amount of detergent for the average commuter isn't enough for us.