Because the market sets the price. You put your rents up above the market and you’ll just get voids as no one will be able to afford to occupy them.
So 2015 rents were at 2015 levels alongside with the earnings/cost of living/bills ratio. If you tried to charge more then, you just wouldn’t have any tenants they would go elsewhere.
Now however mortgages and rents are rising above people’s ability to finance them and afford that same ratio. It gets to a point where half the people in the country cannot not afford the basic necessity of shelter.
https://www.walesonline.co.uk/news/uk-news/half-people-paying-rent-mortgage-25966457.amp
Yes desperate people cut back and will pay more rent and there will be more competition for fewer rental properties but that can only go to on so long and is area dependent. The cushion to finance an unexpected cost like a car service or a household bill becomes untenable.
There is a ceiling for rents (which is LHA on housing benefit). You can fit more people in (HMOs etc) but you can’t charge more and more. This alongside government taxation will force most private landlords out.
What it means in the meantime is that more people will have to start renting out rooms instead to share costs of living - lodgers basically. The tax incentives are already in place. Things like rent controls may come into place once Labour get in (again more pressure on landlords).
House prices will correct eventually in line with historical earnings affordability (5-6 times down from its 9-10 today), but not many will be in a position to take advantage because of the contraction and cost of credit conditions (no more easy options at low rates).
Big organisational pension funds and banks will eventually become the landlords and will pick from the pieces with the likes of people defaulting (essentially porting people over to IO mortgage options is renting) and the likes of build to rent.