The problem with verbal agreements is that while they can be valid, they are far more prone to discrepancies. A landlord files to evict a tenant who had an unauthorized aka ILLEGAL - they mean the SAME THING - person staying, on a verbal, month-to-month. The case is heard in front of a judge. The defendant claims that the landlord said unauthorized, illegal tenant X could live there. Judge turns to plaintiff and asks, 'Did you give authorization?"
"No, I did not."
He said, they said. Unless it can be documented somehow that the unauthorized person lived there openly for months and months with the landlord doing nothing about it, which would then make that tenant authorized and legal due to the landlord's failure to act, the tenant really has no case.
On the other hand, a written lease removes all doubt. What is on paper and agreed to by all parties who sign it IS the deal. If a lease says, no person who is not a party to the lease may take occupancy (occupancy is being a tenant, not a visitor), then it is VERY EASY for a landlord to get rid of unauthorized, ILLEGAL people AND the tenants who permitted them to live there.
And that is why landlords insist on it. It gives them control over who lives in THEIR property. You seem to think an owner of private property should not have control over who lives in their property. Why?