In your family, do you spend the majority of your medical bills on the healthy members of the family or on the sick ones?
The spiritually healthy members of our community care enough about Torah and mitzvos to make them a priority. They will be mosser nefesh for chinuch, shabbos, kashrus etc.
But lo aleinu the spiritually sick of our brethren (who don’t even realize that they are sick) will not extend themselves to send their children to yeshivas, spend extra for kashrus etc.
These spiritually sick will eventually infect us all, so by helping them we also improve our health. Kiruv orgs are not for “them” but for “us” – to insure we remain spiritually safe.
Besides this, arvus and ahvas yisroel keeps us away from the them vs us mentality.