I think you misread the Wikipedia article re:
The Absurdist's view of morality implies an unwavering sense of definite right and wrong at all times
or at least took it out of context. Reading the previous sentence in the article, "The absurdist is not guided by morality, but rather, by their own integrity", should illumnate that.
With that in mind, one can easily extrapolate that they recognize the validity of your question. Right and wrong are meaningless, temporal, and subjective; adding up to absurdity. However, they don't then throw the baby out with the bathwater and become nihilists; the ground of integrity is considerably more solid than morality, and gives one a stable path to walk on under all circumstances. Morality hangs witches; integrity does what this guy does.
And why/how did this particular philosophy corner itself into such a simplified interpretation of things
Where did you arrive at this conclusion? There is nothing at all simplistic about this philosophy. Unless of course you consider something simplistic that does not claim to know or attempt to answer all questions definitively.
Consider this quote from the Absurdism Wikipedia article referencing Camus on Elusion:
filling the void with some invented belief or meaning as a mere "act of eluding"—that is, avoiding or escaping rather than acknowledging and embracing the Absurd. For Camus, elusion is a fundamental flaw in religion, existentialism, and various other schools of thought. If the individual eludes the Absurd, then he or she can never confront it.
That is called taking fantasy as reality. That is called inventing a belief system; a point of view; a scientific perspective; etc. and elevating it to the status of dogma. To embrace the absurd doesn't imply (as I suspect you might be intimating) simply throwing up your hands and adopting a worldview of lethargic agnosticism (in fact doing this would be called eluding), but rather confronting life as it is and as it isn't without inventing fantasies to avoid confronting the absence of known external meaning and the certainty of your own death.
One of my favorite all time quotes comes from a little book called The Pocket Zen Reader, and is attributed to Dogen. It goes:
One need not necessarily depend on the words of the ancients but must only think of what is really true.
Simply put. To attempt to avoid the absurd, shield yourself from it one way or another, is to willingly blind yourself to what is right in front of you. It isn't to say that there isn't incredible and immense joy to be found in this life, but without seeing what's true (I mean this at the most basic level, like I am certainly going to die at some point;or I keep worrying, but I don't know what is going to happen tomorrow;), one invents or subscribes to myths - taking them as facts - and sees life through the filter of those myths.
To embrace the absurd is to see the fantasies you create about reality (to create stories about our lives is a seemingly inescapable aspect of being human) as fantasies; as nothing more than fiction, and then to love the fantasies and be their author rather than being a mere instrument of some novel written by someone else (God?). Seeing life as it is and as it isn't without adding anything or garnishing the future with hopeful thoughts allows one to go beyond the absurd.
The best quote I could find to illustrate the real power of the Absurdist viewpoint is by James Carse:
“What will undo any boundary is the awareness that it is our vision, and not what we are viewing, that is limited.”