Wasn't it mentioned that Striker Eureka having around just a dozen kills was already a record? A main character Ultra (usually) gets to do that like multiple times over during his own show. At least if we count kaiju-sized seijins.
Also, Ultras are probably more cost-efficient for goverments. I don't remember any case when an Ultraman's human identity gets discovered because he was more high-maintenance than his teammates.