Because a lot of new developments designed for the young professionals or city-living families, academic families in the area are a similar price per person for two or three bedroom flats, but there are some huge difficulties and cons arising from those. Deposits are much higher for these kinds of properties, and background checks are much more stringent, I've seen situations where agents have been looking for as much as £35k earnings per year per tenant or even higher for guarantors given the guarantor covers the entire property, and this is in Sheffield, one of if not the cheapest of the larger cities. These organisations just aren't used to the kind of rent:income ratios that students have and they're not willing to shift, alongside many companies straight up not renting to students, particularly undergraduates.
You also have social aspects, a lot of students, especially younger ones, enjoy living with more people and enjoy living around other students. Hence there is a premium that can be asked for these big houses in student areas.
I'd also dispute that these houses are actually as cheap as you say, in my area there was as much as a 150-200% markup on houses in student areas compared to other areas. There are also plenty of other costs involved to get the property to the point of being rentable, desirable, and reasonably student-proof. I'd also consider that the last few months probably aren't the most desirable to purchase a rental property, particularly one which doesn't currently have tenants, and that will naturally result in lower prices.