I also kinda think because The Flash isn’t exactly fresh. The TV show had him teaming up multiple heroes, he constantly goes back n forth to multiple earths, he’s teamed up with Supergirl you can pretty much say he teamed up with Batman as well cuz Green Arrow on tv was pretty much sears version of The Dark Knight.
Random Thought: I still think the stupidest thing Snyder did was kill off Superman in BvS. What the hell is the point of killing off Superman when you have a Justice League movie coming out next year? lol now you gotta waste time and plot just to bring him back.