Among their many many actions they get rid of the majority of S-Class threats before they do anything, out of the remainder organizations such as the Guild and Thanda act against leaving only a few around out of which Endbringers, Nilbog and Sleeper are the only notable ones. The first is one of their main focuses but it is a work in-progress since Endbringers keep revealing new abilities whenever things aren't going the way they want, the second is not actually going to do anything so there is plenty of benefits in leaving the organizations they built handle it, and the third we have little information on but he largely stays put.
The S9 are vastly overhyped by the fandom; they survive by avoiding any meaningful opposition, kicking people while they are down and laying low, the first time they accidentally get into an actual fight the group quickly disintegrates. From whatever point of view you pick whether it be Cauldron, PRT or USA's government entirely there are countless issues higher in priority, the S9 simply are not important, much less defining, like people keep having them as.