I agree, to an extent.
There is always a need for professional landlords and they will probably have a portfolio of properties because that's their business. I can see why they'd expand over time and buy more properties. It does seem to be assumed that everyone wants to buy, whereas some people do genuinely want to rent, at least in the short term. I know I did when I first moved in with a partner - the relationship didn't work out in the long run and I was bloody glad we could just hand back the keys rather than deal with a sale. Equally, I've rented when I wasn't sure whether I wanted to stay in an area in the long term, so to be able to rent while I decided was great.
But that's with the proviso of professional landlords (and, as an aside, I would argue that they (and all landlords) should be subject to licencing and harsh penalties for breaches to get rid of the 'slum' landlord minority). The problem comes with the "We've got a bit of cash so we'll invest in a BTL" landlords who have very little idea of what they're getting into and think it's a sideline that will bring them short-term income and a long-term nest egg, while not wanting to ever have to pay out for anything. Everything is always the cheapest and most low-maintenance, so they don't care if they make the property look crap as long as they don't have to lift a finger. They don't care if their tenants are an anti-social nightmare as long as the rent is coming in. They're full of their right to earn money and none of their responsibilities as a landlord.
As a solution, additional tax and enforced licencing should be brought in that would put off the 'hobby'/slum landlords and ensure that only committed, responsible professional landlords are operating.
(I do realise that there's a 3rd category of the accidental landlord and that should sit outside that arrangement in the short to medium term to allow people the freedom to, say, rent out a home if they're going overseas for a while etc)