PS: I am sure that's not what you intended, but I'd appreciate it if you avoided that condescending tone. My argument doesn't rely on your sympathy, and reframing an argument as narrative doesn't help either. It puts into doubt all good faith you may have in a discussion.
Also, my tone was not intended to be condescending. I wanted to be let know if I was wrong, genuinely. I am eager to update if there are other places where the renters are identifiably and largely YIMBY, because that would break some of my personal beliefs about the housing market. Any my lack of sympathy is for the narrative -- which I've heard many times before -- not you personally. I'm positive that you are great.
Whether it's because increased density would spoil their views, or make parking harder to find, or the social housing would bring in "undesirable elements" or lower their property value, the incumbents have a lot of reasons to oppose housing changes that would benefit society as a whole.
Allow me to introduce you to the acronym NIMBY[0]. Often found in newspapers with their arms crossed looking stern in front of a building site. And you'd be amazed at what they can oppose - kindergartens, schools, even playgrounds (as they increase noise, and there's a risk of balls coming over the fence!)
And planning laws that require authorities to consider submissions from residents are one of their best weapons - after all, it's typically the wealthier who can afford to own property, and so it's often the wealthier who can form groups that hire top tier lawyers to fight developments they're opposed to.
Toronto.
[1] only half serious
Generally home owners are pro regulation to avoid building larger stuff in their neighborhoods though. Zoning and Nimbyism are home owner driven.