It's not about understanding that it's multiple realities.
It's that the general public wouldn't be as interested as much in the same characters.
It simply devalues the character if they appear too often. Batman has had what a few games and a handful of movies. Superman has had less movies and a show a decade ago. And that's definitely on purpose.
But now, they have an audience that kinda grew up, maybe not reading the comics, but watching Superman and Batman and their friends as cartoons when they were kids.
And execs can keep their interest alive with all these side characters that they can use and say "Oh hey, watch this show, this is Batman's friend/ protege/ sidekick/ etc."
There's also the whole money side. Having their biggest assets on tv won't garner much money. Whereas they can use these side characters and it kinda balances out.
Maybe they want to test people's interest in this character or that or this kind of storyline. Maybe they can test out a consistent little universe, which nerds fucking love. Maybe they can test out a new platform (Titans and YJ with the DC thing).
And for all these side-projects, they can use the side characters and keep the big ones for bigger projects.
Of course, they can create a Batman or Superman show, but why would they when a slightly less people will watch a Green Arrow or a Flash show.