The bananas could have been stored in refrigeration system.  If the bananas are put into a refrigerator or stored at low temperatures before ripe, they may not be able to ripen even if let in room temperature later.  To increase the ripening process you can put bananas in a paper bag with an apple or tomato.  This will usually speed up the ripening process.  You can eat green bananas, but they do not taste the same. You can find some recipes on the Internet using green bananas.   Green bananas are high in starch.  As a banana ripens the starch turns into natural sugars which add flavor and taste to the banana.