1/ If one roommate is disruptive (noise complaints, property damage, safety issues), landlords and other tenants have limited legal tools short of eviction of everyone. That blunt instrument makes it unattractive for landlords to allow multi-tenant arrangements.
2/ From a legal discriminatory standpoint, the law doesn't have much protections for people blocking certain raises or genders from renting.
3/ Many local codes were written with “traditional families” in mind. Some municipalities cap unrelated adults per household (e.g., “no more than 3 unrelated people”), which makes normal roommate setups technically non-compliant even if the lease is joint.
4/ Standard renters or homeowners policies often don’t contemplate multiple unrelated parties. Landlords worry about claims, while tenants may find themselves uncovered in disputes or accidents.
I tried to get umbrella insurance for myself, but because I rent out other rooms and I didn't want to also cover my 2-3 roommates, I am forced to go uncovered or find another provider.