As far as the original topic – I think that Progressives (and a lot of people) are enamored with Theory over the the messy real world. They will say “Real Communism hasn’t been tried yet!” In much the same way the CAGW crowd loves their models over the messy and inconvenient real data. Theory is clean and neat and tidy and explains everything. Reality is sloppy, full of “gotchas,” and frequently doesn’t agree with theory.
Being able to keep the fruits of one’s labor may not be the only motivator, but it’s a very big one. It may not be the perfect situation, but to heavily paraphrase Mr. Twain, “Capitalism is the worst economic system ever devised, with the exception of everything else.”