some of us have very "stuffy noses". I frequently get blocked nasal passages, and only by religiously using a steroidal nasal spray can I breath through my nose. I've had to travel to another city just to get my nasal spray on two occasions, when there is literally zero stock in my city (just prior to hay fever season), so access to this is not a given.
Obviously yes, mouth breathing sucks for a variety of reasons, but if you can't breath through your nose it is significantly preferable to the alternative.