Because understanding it means their housing price should go down. If they can pretend supply/demand doesn't apply, they can keep advocating NIMBY policies that increase the price of their house year after year.
Yeah, it's weird. If building more housing will cause the value of their neighborhoods to skyrocket, why not do it and make a killing? Are we to believe that their deep sentimentality about neighborhood character overrides any consideration of the supposedly massive amounts of cash on the table?
I find it best modeled as a collective action problem. SF in particular, and California in general, has handed out vetos on housing with a quite free hand.
There are a great many people who have no particular reason to want to change their surroundings. Especially when they are keenly aware of how many of their neighbors will happily shoot down any effort they might make to make a killing.
In short, the massive amount of cash is realistically not on the table in the eyes of many. What they do have is civic pride, property taxes decades out of step, and an ever-appreciating asset. All of which they can reasonably expect to pass on to their children.
So no one wants to enormously increase the value of their ever-appreciating asset by allowing for more housing to be constructed? Because of... civic pride? Really?
I'm a homeowner in a desirable area. In my area I see the urge to veto upzoning and keep the riff-raff out, keep traffic from getting worse, etc. pitted against the desire to increase affordability by upzoning. That opposition of interests makes sense. The idea that I could make a killing from upzoning never occurred to me. It seems very implausible.
People willing to sell and leave are happy to consider it. People who don't want to leave in the near term are very interested in keeping the riff-raff out and thus in fending off structural changes that would let you make a killing. Thus all the vetos that make it easy to keep your neighbors in line should they start thinking about that money. Accusations of greed destructive to the community are easy to level and rarely require much in the way of substantiation.
Upzoning is both a way for you to make a hefty chunk of change and an affordability issue. It's often more difficult to impugn the motives of people seeking to improve affordability, however.
Look at it in numbers, though. A million-dollar property on a large lot could become two duplexes, each unit $300k-$500k. Or a six or eight story building with multiple units on each floor. There's money to be made there for the likes of you and me. Neighbors who don't get a cut but want to keep the riff-raff away would like us to not consider it.