I don’t know how things work in Greece, but in the US the process of mortgage foreclosure and eviction takes way more time than evicting a renter who is no longer paying rent. In other words, the bank is already willing to wait longer than a landlord for a delinquent occupant to resume payments.
After foreclosure a bank does the exact same thing as a landlord: they look for a new occupant, i.e. offer housing to someone who needs it.
Banks most certainly do not “hoard” the property. In fact, banks are usually far more impatient than private individuals who want to sell their homes. The unwillingness of banks to hold real estate is another reason why they end up selling it at a discount.