Air breathing saves wet-mass, but increases dry mass. So far, the dry mass increase has been enough to make it not worthwhile. Especially since the change in trajectory required results in much higher drag losses, and more worse thermal performance (rocket gets hotter)
With the notable exception of anti satellite weapons which have often been fired from military aircraft. It’s cheaper because an individual flight on an existing high altitude high velocity aircraft is cheap. Unfortunately, it would require an enormous number of space flights for this first stage to be worthwhile otherwise.
