DMV agents are probably not very wealthy. By suing them you would simply drive an underpaid worker into bankruptcy and probably never get much money back.
Meanwhile, the DMV can easily scapegoat it's employee and never reform or make any systematic changes. Furthermore you would have no recourse to sue them directly since they can just keep hiring more poor workers to be thrown under the bus.
Ultimately the tax payers (or voters) need to keep the DMV accountable. There is no alternative. Democracy doesn't have shortcuts. The tax payers have to pay when the government screws up. More to the point - the tax payer ALWAYS ends up paying when the government screws up, without exception, 100% of the time. Either they pay by having a corrupt DMV that hurts society and everyone at large, or they pay through lawsuits and higher costs at the DMV.