I haven't had a chance to read the whole thread, so excuse me if I'm stepping on another commenter.
I believe it's because they currently work for the ultra rich and corporations. This was not always so, but it most certainly is now. They answer to no one, and they have their constituents utterly in the fold. There are buzz words and tones, and boom off they go. But actually it's always about funneling more to the top. The Democrats are essentially everyone else. Not nearly as cohesive. Not nearly so monied. The GOP lie, because they can. Only the rank and file can demand honesty, and they won't because they've been groomed to hate "the other" and buy into the narrative.
They are so consistently dishonest because it works.