Mortgages were, prior to assholes screwing the whole thing up with mortgage-backed securities, seen as one of the lowest risk things banks could handle.

If you default on a mortgage the bank forecloses and auctions the home. This was QUITE rare before the housing crash. The problem was that the banking industry became so lax that they were giving loans to people that actually did NOT have the money to pay for them, figuring that they could just seize and sell the home as they always had. The problem THEN is that mortgage-backed securities were a thing by that point and every foreclosure caused another domino to fall over.

It became a shitshow because banks fucked themselves over being greedy pieces of shit.