I just don't see the point of doing the Crisis if the result isn't all the Earth's merge into one. I mean, that's the whole point of the Crisis.
That was the point in the comics, but an adaptation doesn't have to serve the same purpose as the original. In the comics, the point of Spider-Man's inclusion in
Civil War was to lead to the exposure of his secret identity to the world and his falling-out with Tony Stark and departure from the Avengers. The movies handled it in a completely different way. In the X-Men comics, "Days of Future Past" was just a basic "prevent a terrible future from happening" story and ended up with the status quo preserved, while in the movies, it was about going back into the past (relative to the audience, not just the time-traveler character) and rebooting the whole series continuity.
As I've been saying in another thread, the point of an adaptation is not just to repeat what the original already did. That would be redundant. An adaptation is more like an offspring -- it has its parent's genes, but it's a distinct individual and may or may not resemble its parent that closely. It's a new story inspired by the original, a different variation on the theme. That's especially true in comics adaptations where you have decades' worth of ideas to draw on and can put the pieces of many different storylines together in new ways.
Besides, we know in retrospect that merging the Earths
was a mistake. DC ended up regretting it and eventually undoing it, restoring the multiverse. There is absolutely no sense in expecting today's creators to slavishly repeat that mistake. We should know better by now.
Bottom line, the responsibility of the shows' creators is to the shows they're making now, not to the comics that were made decades ago. They have to tell the best stories they can in their series, with their characters and plotlines. We've seen countless times already that when they take ideas from the comics, they don't slavishly copy them, but they use them to serve
their own characters and plots. The comics are not a straitjacket, they're just a source of ingredients to mix together in a new recipe.