Someone on the margin of affording a rental in SF isn’t homeless. They live someplace else that costs less. Why should taxpayers foot the bill to build more homes with the goal of making SF marginally more affordable when there are plenty of other good places to live? How is that better than some dumb stadium idea? And how long does it keep housing costs down? Those new homes will be gobbled up immediately and then the available inventory on the resale market will be back to about the same level as before.
Because the the best jobs are in SF, so scratching SF off of your list if you're in particular industries is too large of a sacrifice.
The problem with NIMBYism isn't saying no to building more homes per se, it is saying no to more homes while saying yes to more jobs and more commerce that creates the problem.
Someone who works in SF, but lives 40 minutes away due to cost, should be considered a constituent of SF and their voice is as valid as that of a resident.
Yeah the city certainly benefits from their presence even if they don't pay property tax. NIMBYs are happy to profit off this as their property values soar, but they don't want to pay any of the cost.
By fighting high density projects that are residential ("It brings too much traffic! It'll shade this nearby park!") but not when they're for commercial use. See recent SOMA mixed use construction in SF. Office to residential square footage ratio is completely skewed towards offices.