hmmm, Good question.
In this case lies are a symptom, not the disease.
DUmmies lie because they are despicably evil at heart. They worship an all providing govt. They tolerate no dissent. They hate Jews and Christians, but love murderous muslims.
They worship the lie of socialism. Deep down they know its a lie, and it burns them: So they lie to themselves. Read their posts and you can feel the burning hate just beneath. We've documented the many times they expressed a desire to kill their enemies. If they ever got power, they would start a bloody purge.
DUmmies of course, are just the fringe element of the left. But. They are also a glimpse into the future. DUmmies verbalize what the left keeps silent. A shrewd man can read Kos, or any leftist website and see the same underlying threat of tyranny and mass murder. The other sites are just better at lies than DU.
If I have to answer the question,
DUmmies lie because they are monsters, and they cannot face the truth in the mirror.