Barium swallow can catch the reflux when it's happening and also show swallowing anomalies/dysmotility - all of which endoscopy can't do. It's another, different, valuable tool.
Denise is right. If the PPIs are working you may not have esophageal damage that can be seen from the inside. I didn't either.
What the tests mostly don't show, unless there is an obvious wide
open LES or paraesophageal hernia, is the cause of the reflux.