And new housing, .. but not built in their neighborhoods. Oh, and don't build over farmland or undeveloped green spaces.

Without being snarky, the problem here is that there is a tension between what people believe in general, and what they believe about their situations specifically, especially when they have an incentive to feel the opposite way about it. For example, people may know (or believe) that building duplexes and apartments in their neighborhoods will reduce their property values, or "ruin the character of the neighborhood" and so oppose them.

Private property developers simply dont want to build affordable housing. The margins are just lower than they are for luxury housing. Who would choose to make 4% profit when you could make 20?

They do lobby hard to remove building regulations in the name of building more affordable housing but they never actually build it unless local government actively forces them to with some of those regulations they hate, at which point they build the bare minimum and install a poor door.

They're always going to build the highest margin product for which there is demand. You're not going to get cheap housing until there is abundance. It's simple supply and demand.