I'm sure that were any council to buy some land and then declare it buildable on, that someone else (a land owner or developer probably) would complain that they were being diddled out of something. And quite likely they would have an argument that it was some sort of abuse of privilege.
And I wouldn't be surprised if having created more buildable land, council found it had to be sold "at market price" with the effect of inflating its cost such that only a developer creating 'executive housing' would be able to buy and Thu having little ameliorating effect on housing affordability.
This example council could create a lot of land for building at once. Which ought to depress price and let other developers in. But would it be enough that lower priced housing occurred,or would it all get bought up for executive homes with garages for two cars and a hanky-sized garden?
Add in the complaints about over development by owners of existing housing nearby, and who would want to be a councillor, or housing officer.