Hard to really say, but I would offer this --
Modern society is built upon worldliness, vanity, and pride. We've built our own Tower of Babel in our vast societies of "heaven on earth". We try to make ourselves happy by buying things. We try to make others happy by funding big government programs. It's all empty. Very empty. So many of us are alienated. We have trouble with friendships in the modern world. Many families are deeply fractured -- but still we persist in an unnatural level of self-esteem and pride in all we have, and in all we think we have accomplished.
If anyone starts to say "The emperor has no clothes" or "You're living a lie" ... well, then ... that just might bring the whole world crashing down around our ears. That would be ... uncomfortable. So, please, don't use the "L-word".
You make an interesting case, but it could be as simple as the fact that a taboo against calling a lie a lie benefits liars and disadvantages the honest.
Dims rely on lies. The Father of Lies is the source of most of their policies and methods. Of course the left would want and set about creating such a taboo.