The short answer is because intellectuals do not think intellectual discourse is serious.
And they do not think intellectual discourse is serious because they do not think that anything is serious, because they are atheistic nihilists who do not believe in reality and who will soon be dead, anyway, so why bother?
Inter alia modern intellectuals do not believe in the soul, or life after death, or the transcendent or the supernatural.
(And if an intellectual does happen to believe these things, they are anyway excluded - both implicitly and explicitly - from modern intellectual discourse and cannot influence its structure or processes.)
Intellectual retreats in the face of to serious rational discourse:
1. Who knows? Nothing is certain. Subjective factors interfere everywhere. It is dangerous to claim certainty when none is to be had. Maybe you are right, deep down, but as we never can know reality then we will never know for sure... In the meantime I might as well enjoy myself, or at least avoid unnecessary suffering.
2. Nothing is really real. Well, ultimately nothing is real anyway; so whether you are more correct than I am doesn't really matter, because the universe is ultimately chaos; any apparent order is just a random blip on the road to entropy. In the meantime I might as well enjoy myself, or at least avoid unnecessary suffering.
3. It will last my time. Maybe you are right, maybe my views will lead to disaster, maybe the current behaviour is not sustainable or will destroy itself - but all that matters is that things keep going just long enough until I am gone. After all, when I am dead I won't know anything about it either way. In the meantime I might as well enjoy myself, or at least avoid unnecessary suffering.
4. Death will put an end to it anyway. Even if things do not last my time, even if things do become awful - even if they become intolerable - then I can always escape by killing myself. In the meantime I might as well enjoy myself, or at least avoid unnecessary suffering.
Atheism and nihilism therefore serve as a crutch to the intellectual elite.
A lot of people nowadays have a lot invested in their belief that death ends everything, and that therefore in the meantime they might as well enjoy themselves, or at least avoid unnecessary suffering.
Naturally enough, intellectual discourse under such circumstances is shallow, flippant and reckless.
After all, why not?