Separate leases change the risk profile
A house rented to one family on one agreement and the same house rented to four unrelated people on four agreements are different propositions to an underwriter. More occupants, more turnover, shared common areas and no single household responsible for the whole property. Several carriers decline the second on a standard landlord form, so the arrangement has to be declared at the quote rather than discovered at a claim.
The academic year creates predictable voids
Turnover concentrates at the same point every year and a property can sit empty through part of the summer. That interacts with the vacancy conditions on the policy, which restrict cover after a stated number of consecutive empty days.
Each tenant needs their own renters policy
One tenant’s policy covers that tenant. It does nothing for the others and nothing for you. Requiring cover in every lease is free, it moves real exposure off your policy, and in a shared house it is the only arrangement that makes sense.