Why do you think they're out of reach in the first place?
Cheap credit, 100%+ mortgages, tax incentives etc. made BTL an overly attractive investment for a certain generation. Once the money poured in, prices started to rise. The more money people made, the more other people wanted in, the more money people made, the more they invested. And so begun a horrible cycle of ridiculous house price inflation for those with skin in the game. If you weren't already on the ladder, you were stuffed.
If you couldn't afford to buy by this point, you had to rent. You know what landlords and agents love? A captive market. People literally had (have..) no choice but to rent if they want a roof over their head. Meaning in a previous generation if they could have bought that two bed flat with a small deposit and paid £1,000 month they would have done so. But now they can't because they can't save the required £50k deposit, so there's a whole bunch more people trying to rent that similar two bed flat. What happens when there's 10 people putting in rental offers on the same flat? Surprise surprise, the rent will go up. And so begins the secondary cycle from this whole affair. You now have two whole generations that can't really get on the housing ladder and are forced to rent. Rents continue to go up, far higher than equivalent mortgages. And in doing so, further locks them out of the housing market.
Does that help explain how we got here? Add to that general landbanking, right-to-buy, tax dodges by foreign investors, a weak pound encouraging even more foreign investors, heck even council tax discounts for second homes. Need I go on?