That's absolutely it. People not only are unwilling to sell - as in their right - but also insist that everyone else be unwilling to sell. If they are willing, the permitting process is made to be as painful as possible, in an effort to discourage everyone involved.

Not necessarily true. An upzoning will likely increase the value of the land, which will result in higher property taxes (assuming this is a normal city with no prop13 nonsense).

The higher taxes could effectively force a land owner to sell their property.

Often part of what existing land owners are fighting for is preferential zoning which artificially keeps the "highest and best use of land" a low density detached house, which keeps land values relatively low.

This is assuming no or poor homestead exemption.