Major difference between private landlords and Tescos is that Tescos don't refuse customers whose income is a low paid job or on benefits. They don't insist on only selling their products to people earning a specific income multiple. They also don't ban people from their shops for making justifiable complaints about gone off/other valid product issues.
Landlords on the other hand do discriminate against the poorest and most vulnerable and refuse to let to them, and the few who do will evict if a tenant has a valid complaint, eg. repairs request.
That said, I'm not "down on all landlords". Some are decent, and absolutely there's a need for some. Not everyone is ready to buy. Young people yet to settle in a particular area or job, students, people on temporary work secondments in away from home, people in-between house moves etc.
The issue though is private renting is inappropriate for long-term living in the majority of cases. So, older people, families ready to settle down, disabled people, etc.
It's also completely inappropriate for the poorest and most vulnerable. Unless housing benefit was increased to meet market rents (and/or rent controls and assured tenancies). However that would be more expensive than building more social housing, and also doubtful that many landlords would stay in the business if rent capped assured private tenancies were reintroduced?
So, whilst you're right that some private landlords are needed and not all landlords are bad, there is a need for more social housing across the UK asap.