Well you start with a story about a hero, and it makes more money as a serial so you never really tie-off the loose-ends
Then it starts to feel dated so you do a reboot every few decades
Then the material is getting a little tired so you do some "what if" and period and role-switching stories to keep it fresh
(and don't say they should have just made new characters. Besides the fact that Superman's name = instant sales, there is a big emotional difference between "What if Superman landed in the USSR?" and "What if this new-similiar-but-not-actually-superman-guy grew up in the USSR?" the DC heroes are iconic, there's no reason to write whole new characters unless you have an actual new archetype you want to explore)
The three points above are all straight-forward and reasonable
And then you say, well, what if the original "normal" superman met the soviet one? That would be interesting (this may not be an actual example because I don't actually read comics) How can we make that happen? And you say, oh they can sort of hop between dimensions/continuities sometimes for fun. And that leaves you with a multiverse, and that's really not too confusing or unreasonable if you ask me. (a quick trip to a dimension where everything is just a little bit different is pretty standard sci-fi fare)
What gets crazy and stupid is when fans start asking "Well what one is the real one?" Take the beginning of the video: "Well if that Flash was real, and that Flash was fictional how can they both hang out with Superman?" None of them are real you dorks, they're all fictional! That is part of why they are fun, you do not have to worry about silly things like what the hell Batman is doing in 1895, you can just roll with it because it is cool! Trying to tie 60 years worth of everything every author has ever written about a character together into one coherent lifetime is completely bat-poop insane, people. Clones and time travel and everything...why bother?
Let the stories stand on their own. Whichever iteration is your personal favorite is "the real one" for you.
And on a related note: I think it's blasphemous to switch authors in the middle of a story. Can you imagine a novel doing that? As somebody else said, every author should get their own earth# in the multiverse. (cross-dimensional stories could be collaborations, or only canon in the earth# of their author) If an author doesn't have enough new ideas to warrant their own earth# then what the heck do we want with this concept-less hack anyway? (I guess a group of authors could work together on an earth# if they shared a vision and collaborated a lot, that's not the same as switching authors.) No-switches would also force stories to end when the given author got bored/fired/dead. This would be healthy for the quality of any given storyline, as dastardly so eloquently explained.