Supply shortage causes house prices to rise, and means fewer people can afford to buy (particularly in the south) so have no option but to rent. This increases the demand for rental properties and at the same social and private housing is not being built. This creates huge demand in the rental sector and causes rents to rise, which then also attracts property investors and BTL landlords.
The rental and house buying market are intrinsically linked. If there was a significant rise in the number of rentals without the demand, rents would be falling. Investors wouldn't be investing if there wasn't demand, and it's the shortage that creates the demand.
The size of the private rental sector has increased from 10% in 2001 to around 20% in 2016 and has stabilised at around 19% since then.
This significant rise in BTL landlords has had a distorting effect on the housing market, reducing availability (to buy) and pushing up prices.
And tax payers are facing a huge bill when the renters eventually retire and claim housing benefits, when they would previously been able to buy their own home.
The only solution is to build more houses, in particular social housing with affordable rents. With more supply, investors will not be able to achieve desired rental income so prices will drop or they will sell freeing up more homes to buy.