G4s were 1999-2003, plus a little longer because Apple kept selling G4 towers for people who needed OS 9 bootability.
G5s were only 2003-2006, and not only that but the Intel transition was announced in June 05 so I bet they didn't sell too many of them in that last year.
A rough estimate indicates that there are just under two G4 towers for every G5 tower. Given that G4s are more out of date with a lot of them being obsolete while all G5s are still fully supported in 10.5 I think that explains why you see so many of the former compared to the latter.