Because one needs to consume housing, regardless of financial status. There are basically two ways to consume housing, to buy or to rent. Buying and renting are easily substitutable strategies and thus by basic economics the prices are equivalent except in speculative bubbles (renting is cheaper) and in panic crashes (buying is cheaper). Since the prices are generally equivalent it makes sense to buy a house if one would prefer to not be subject to landlord-externalities and/or if one would prefer to own one's housing outright 30 years in the future, when retirement looms.

None of this has anything to do with student loan debts. They are utterly irrelevant with respect to whether one should choose to buy or rent housing. It only makes a difference as to how much housing one can consume.