Building control and planning departments are massively understaff. Some staff are probably "captured".
Developers can outsource building control to private companies. These private companies are at risk of getting less work if they tell the developer that the development does npot pass building regs.
I once looked at a house for sale and tried to buy it - the survey picked up that there was not building control approval for removal of chimney breasts or complete rebuilding of the entire back wall of the house... but the conversion of the cellar had been done to building control standards DESPITE NOT HAVING BEEN DONE AT ALL!!!