We're born into presumptions. We grow up surrounded in presumptions. We live through a life full of presumptions. We die still surrounded by presumptions.
I think it's safe to say that most people act as if those presumptions are solid borders that one can't – "not allowed to" – cross. Behaving in a way that others see as silly, or out-of-place, or not based in reality challenges others' presumptions, and we're terrible in handling views that contradict ours, even if said behavior does no damage to the fabric of the society the "silly sod" is in. (Your comment's upset tone suggests as much.)
Imagine seeing a man dancing down the street. He doesn't disturb anyone, takes precautions to avoid colliding with people and things. He has no headphones on. What would you think of him? That he's crazy? mentally-ill, perhaps? unstable? Maybe he's drunk? high? under a different kind of influence?
He's just dancing down the street 'cause he feels like it. No one is hurt, yet so many are upset. Why? He's challenging the presumption that it's not okay to dance – or express joy so openly – in front of those uninvolved, in front of strangers.
Why do we presume so? I haven't the slightest clue. (I have an idea or two about why it could be beneficial – social coherence being one of them – but I don't see any ultimate cause for such a barrier.) Just because I might relate to how the majority would feel does not automatically confirm that my feelings are meaningful, in an existential way.
No, just 'cause someone told you something is something, doesn't mean it is. If so many people arrive to the same conclusion independently, however, it should say something about the validity of the state of things. I don't need people to tell me how to feel – but I feel similar to how they feel, maybe we're onto something.
A more productive venting for your frustration may be asking more insightful questions. Asking what the idea of "absurd" in "absurd life" derives from is a good start, but only as long as you seek to hear the answer, rather than have it disproven.
For what it's worth, I find that the reasons you state in the parethesis are of little relation to the reality of such views. They seemed, intuitively, to have paved the way for people's disillusionment, which probably sparked a whole lot of introspection and consideration. I don't think, however, that people never wondered the same questions before. (Might be wrong, but hasn't Marcus Aurelius famously expressed similar views in Meditations, at the time of Ancient Rome? I need to finish that book.)
As a matter of fact, I haven't seen a single philosopher denouncing existentialism in favor of the more "real" reality, for things matterring exactly as much as we value them. (I'd love to be proven wrong: that would be a marvellous read.)
Have you never felt like an unwritten rule in the society around you made no sense, regardless of how much pressure you received towards following it? George Carlin said, early in his career (paraphrased):
> Not running with scissors – that's a good rule! That's a rule I'd like to follow! These motherfuckers are sharp! Not singing at the table, on the other hand... What happened there? Some poor bastard sang badly at the table, and the rest of us have to suffer for it? What about singing while standing next to the table?¹
¹ https://www.youtube.com/watch?v=2pgzWRVEqHY
The definition of "absurd", roughly, is "not following what is evidently true". You asking me what time it is and me replying "Chicken!" is absurd. So are so many of those presumptions we're born into, live through, and die with. I don't think it's because we die: I think it's because most of those presumptions make not a lick of sense, when you think about them.