Plus, as is probably still the case in a few parts of the US, a given mortgage may well be underwater (i.e. the house, even at market value, may be worth less than you owe). As others have pointed out there are also a lot of transaction costs, especially for a house (don't forget moving/storage/etc. too). If you've largely paid off your mortgage, sure, but then you have a great source of collateral if you need some short-term money.
