"sorry, I thought that's why we elected them."
Then you thought wrongly.You elected your MP to represent your interests in the House of Commons.
Some MPs are appointed to parliametary select committees, and when the committee is sitting it acts (or should act) in the public interest. Parliamentary committees do not have the power to compel a government Minister to give evidence, and in fact they don't have the power to compel members of either house to give evidence. That Ministers and MPs do appear before the committees is a purely voluntary thing, and in the main they do it because they believe in the principle of open government.
From time to time some members of some committees seem to get a false impression of their power, and they push things. It's ridiculous to expect a Secretary of State to provide detailed information about the day to day business of a government department. What the committee can do, and should do, is ask a Minister to justify policy decisions after the event - in other words, to account for the consequences of his or her actions, if revealing such information would be in the public interest.
We elect people to run the country on our behalf, and the assumption has always been that they will act in our interests as they do so. We shouldn't expect to be informed about absolutely everything they do, unless it goes wrong, in which case we can reasonably expect to be able to hold them to account. That's what we have opposition parties for - their job is to harry the government and expose its flaws.