You skip over a very important step here, where people keep buying the MBSes because the ratings agencies are knowingly rating the securities incorrectly. If that didn't happen, the market would be too small to blow up in the way that it did, all of the safe money can't invest if the MBSes aren't AAA.
It's not that no-one noticed in time, it's that the people responsible for noticing were paid to pretend they hadn't. That is the corrupt part.
What they were doing was, they'd take a bucket of high risk mortgages and apply a contract to them to retroactively sort them. So, if you bought the 30th percentile of the bucket and then anything more than 70% of the people in the bucket paid their mortgages you would get paid, and if fewer than that did then you wouldn't.
Then they were rating the highest percentiles in the bucket as AAA because even for borrowers with bad credit, the probability that such a high percentage of them would default was considered very low. Even for people with bad credit, default rates are usually only something like 10%.
But that doesn't work out if you haven't noticed that banks have stopped caring about the default rate when issuing mortgages.