Many people saw that housing was probably overvalued and that there was a wave of unaffordable payment hikes in the pipeline on the loans secured against that housing coming. A falling housing market is hardly a once in 100 year phenomenon, and many other housing markets had operated with similar products (teaser rates etc) for many years, and continue to operate on that basis.

Many people also saw a recession coming, because recessions always come eventually - it's a normal part of the economic cycle.

Relatively few people saw that these two things would result in many of the largest financial institutions on the planet coming near to collapse.

Even fewer people got the timing right. And not many of those had the courage of their convictions to actually place bets on it.