There's also the idea that if a character is an orphan, the story is usually more free to go in multiple directions.

WHAT?! WHAT DID HE SAY?!

Wait what's the bench thing?
Edit: I found it.

Or the game

I was the Anne Frank, can confirm

Thirteen
Fifteen
Seventeen
Nineteen

You have my parmesan to continue

It's not on Netflix though

Recently though, they've been more willing to do it now, seeing as they let Superman in the Arrowverse, as well as a potential Bruce Wayne (not Batman) cameo.

@kolorblind It's mainly due to scheduling problems on if actors are available. For Grant Gustin (CW Flash), he's the main character of the show and can't really have much wiggle room. There's also the part of DC not allowing it to happen because they don't want to confuse the audience. It's the reason why they disbanded the Suicide Squad on the TV show, that being not wanting two of them in different continuities.

I'm gonna bring a baguette with me to classes instead of erasers now

Or a shirt found from digging holes