When it comes to the Arrowverse, I kind of assume the heroes all have some kind of an agreement that they'll let each other deal with any threats local to their city, and only come together for world/universe level threats.
Why? Wouldn't it make sense to team up when the threat is serious? And for what it's worth, we have seen global level threats that for some odd reason, didn't have other heroes involved.
It makes sense that for one on one issues, one hero, one villain, there doesn't need to be team ups. It isn't called the never ending battle for no reason.
But on these shows, clearly there were threats that endangered lives where a call to another hero was important. Black Lightning last season is a perfect example of that. If Jefferson called Clark for example, he would have come.
As noted in the Black Lightning threads, the showrunners had their season plotted before that CW-COIE, and its natural development would have been upended by other characters suddenly showing up, particularly characters who would not fit the world Black Lightning established (fantasy framing serious real world issues). In short, Black Lightning was better off and finished its season as its own universe.
That's not an in-universe explanation. The fact that the writers wanted it that way doesn't make it make sense.