because there are people with mums and dads willing and able to cough up a few hundred thou' and there are prosperous people who like to invest in BTL.
"According to the latest ONS data, the number of affordable homes delivered annually has barely shifted over 10 years, despite rocketing demand. The government figures show that there were 60,000 affordable homes supplied between April 2017 and March 2018."
Interestingly, the very profitable housebuilding companies like to hoard land and dribble it onto the market so that house prices remain high due to restricted supply.
In other news, housebuilding companies donate large sums of money to the Conservative Party.