Maybe the writers were trying to be more realistic than the comics, where the heroes are often completely isolated. (Green Lantern Corps isn't that way, but we haven't seen them show up in the ArrowVerse TV series yet.) Maybe they're trying to save money on the special effects. Or maybe they use teams because that's the way they write every other non-ArrowVerse TV show. (Modify standard cop show template until you get a superhero show template?) Who really knows?