cos you don't have to deal with your flatmates boyfriend inviting himself to live with you for a year without paying anything towards the rent, electricity, council tax, gas etc etc etc, despite you asking your flatmate specifically before you moved in "is your boyfriend going to invite himself to live with us for a year without paying anything towards..."
Or him trying to climb in a first floor window when he was pissed, falling off the drainpipe and breaking his ankle, and then spending three weeks lying on the sofa.
or for one flatmate to start sleeping with another flatmate's ex-boyfriend
or for another flatmate to start selling crack and running an escort agency from the downstairs room.
these problems just melt away when you don't have to deal with other people.