It's less about the self-serving approach which I've seen on rare occasion. It's more the complete inability to estimate magnitude effects. If you found a way to replace capital entirely but keep all the efficiency of price mechanisms you could maybe give people a 10% raise (but I bet whatever mechanism replaces price loses the entirety of that 10% due to inefficiency) but the average marxist will confidently tell people they could double their paycheque if the greedy capitalists weren't taking half.