Why are politicians not accountable for their actions unlike every other private or public organisation?.?

Im amazed that politicans who make decisions that cost the taxpayer billions of our hard earned money are not held accountable when their decisions are found to be completley wrong,eg the NHS computer system.If this were a private company and the board wasted billions of shareholders money heads would roll.Not to mention all the expense fiddling going on,if this were the private sector the police would be involved or is it not a crime if you are a politician and they are exempt .