Good and bad are elementary concepts that we should just throw away when talking about politics. Politicians seem dumb because like you said, they cannot know about all the issues a person can know about. If they do, it's usually not at an expert level. So they listen to people who tell them things - more correctly or less. And they do things that they may think are right, but majority may disagree with. I honestly don't think the current system is designed to accommodate societies of our size and no politician entering the space can fix it. Also, some, like you pointed out, realize they are just a cog in the machine and that the machine is far too large to do anything about - so they end up either getting corrupted or just going along with the flow to milk the job. 

Of course some have good intentions, but once you realize there's little you can do, and that perhaps you don't have enough knowledge about any given subject to make the best decision, you kind of just let it unfold to the best of your ability - which we see as incompetence.