For example if you gave software developers a bunch of cash and told them to work on whatever they wanted, how many would go and build video games or invent esoteric programming languages (fun but not important) vs how many would want to build (or worse maintain) a system for the administration of taxes (less fun but very important). Hell, how many would even develop software at all vs just take up some other activity of even less value.
Therefor the best way to motivate these people is to extrinsic motivate them with money. However what happens if most of that money is taken as taxation and spent on people who sit around and do whatever they want all day? Do they still have the same incentive to do the boring but necessary jobs?