Well, having no tenant certainly does not give you a guaranteed return.

When there's high demand this doesn't happen much. A bigger problem - so I'm told - is the cost of wear and tear, decoration, and repairs after tenants leave.

As for landlords, owner-occupiers traditionally rent out spare rooms anyway. So owner-occupiers don't necessarily push rents up. Generally they're less likely to be demanding about rent because they don't have to pay the extra cost of repairs/maintenance/redecoration. And they can choose to rent out casually, or not.

BTL landlords don't have that choice. They're on the hook for all expenses, they don't live with tenants so their costs are likely to be higher, and they have to pass those costs on tenants to keep up with payments.