There are problems in the planning system, certainly, but at the moment we need an undeveloped land tax to stop this behaviour.
Sources: http://www.theguardian.com/society/2015/dec/30/revealed-hous... http://www.local.gov.uk/media-releases/-/journal_content/56/...
Not just this but also because they still need capital to finance the construction process. Borrowing short term is a model that works for them when demand is high - they don't want to be sat holding empty properties after all.
Longer term finance, allowing houses to be built for rental, is one solution that's slowly gaining traction with pension funds and the like.
Of course, as you say, the status quo suits the major housebuilders just fine so there is a lot of inertia when it comes to adopting new and innovative business models.
Seriously, it may be hard for young people to enter the housing market. But no-one wants to go for a solution that leaves the people who do own houses today with mortgages bigger than what their house is worth on the market. You don't help 10% of people by screwing over another 70%.
"The vast majority of the 475,647 homes quoted by the LGA are either on sites where work has already started, or where there is not a fully ‘implementable’ permission and where it is not legal for builders to commence construction"
http://www.theguardian.com/society/2016/jan/07/number-of-unb...
"To get to 475,000 unbuilt homes, the LGA have looked at every development scheme that has permission, and decided that, until the scheme is completed, the whole development should be classed as unimplemented – even if most of the houses are mid-construction, or even if they have been finished.
If you count houses that have been built as unbuilt, you are obviously going to end up seriously exaggerating from reality. I am writing this article from a flat that was finished 18 months ago; but another part of the development is still being built. According to these figures, I live in an unbuilt home."
http://www.citymetric.com/politics/according-lga-my-flat-doe...
Also, it's pretty bad form to ghost-edit your comment after it's been commented on.