I understand what you are saying, and in a perfect world we'd see these other characters. But we all need to realize there is still a pandemic going on, and all of these shows are having to isolate themselves while filming. It makes crossovers incredibly difficult to do, and so they didn't do them this year (though they really should have included that picture of Kara, if nothing else).
This is also the same argument I've heard since Thor: The Dark World, when people were asking why Thor didn't call the rest of the Avengers in to help with a potentially world-ending threat. That works in a comic; it's unfeasible in film. If they could do that, then Oliver could have contacted Kara to help rescue everyone off of Lian Yu at the end of Season 5, or contacted Barry to help him take down Diaz during Season 6. With contracts, with budgets, with schedules- it just won't happen.
All that said, I know they are working on a crossover of sorts for the beginning of The Flash next season. So hopefully with the easing of Covid restrictions (and if there aren't any more major spikes), integrating Superman and Lois more into the Arrowverse at large should be easier.