I have rented here for 20 years.

When I arrived, a single leaseholder for a shared apartment was normal and had been for decades. With the same rules on eviction. It worked because the LL didn't want an eviction and neither did the tenant. A whole apartment could be placed quickly and did/does, when you aren't charging 4500/month.

Someone sold you a bill of goods or you're lying about being a renter.

Small landlords aren't doing this because they'd have to pay someone to act as a leasing agent for each room. This is time consuming and expensive to find many people for one apartment. To take on the additional responsibility of managing the interrelationships within an apartment? A tenant with a lease has obvious rights and they can exercise those even more so when they live with the person. It's only become popular as building have been bought up by hedge funds and have property management companies to handle this.

Please stop with your "think of the poor hedgefunds argument." The only thing multiple leases did was drive up the rents in various popular neighborhoods.