1. One of the things Prop 13 does is to cap the maximum rate at which a property tax grows per annum to 2%. However, due to housing demand, the property values grow at a much faster rate (7% is not unheard of). The property tax can change when there is a sale.
2. Bay area prices have skyrocketed.
3. The peninsula topography and zoning laws make it hard to expand housing capacity.
So, if you are an aging home owner, sure you can cash out of a sale. But where would you go? Unless you can afford another place in the same area, existing home owners do not have incentives to sell their property, which results in a very low inventory. And, low inventory further puts pressure on the prices and drives it up.