It's because planning regulations are such a pain in the arse. When they get the initial planning outline for a plot of land, that is a huge multi-year hurdle

It's because planning regulations are such a pain in the arse. When they get the initial planning outline for a plot of land, that is a huge multi-year hurdle they've overcome. It can cost hundreds of thousands, even millions, just to get to that point. So naturally they then decide to cram as many houses as possible onto the plot. There are also really annoying regulations about having to give a certain percentage of the houses to the council (on the cheap), if the total build is more than about 12-13 houses. So when you're choosing between 200 or 300 houses on the plot, why would you want to give away some really nice houses at cost, when you could just build a ton of small terraces?