The difference is that shareholders lose something (the value of their shares) when they drain a company of its assets. This might still be short-sighted, but there is at least a balancing force.
In public services, where making a loss is o.k. this is just included in the loss, and there is essentially nothing that encourages the 'parasite' to not drain the system.
In public services, where making a loss is o.k. this is just included in the loss, and there is essentially nothing that encourages the 'parasite' to not drain the system.