It's the saying doctors learn in medical school. "When you hear hooves, think horses not zebras."
Probably 99 out of 100 people with GP's symptoms (if not more) actually do have acid reflux. It therefore makes medical sense to assume that until additional symptoms indicate a different conclusion.
> Yeah, pancreatic cancer patient here. Mine presented as acid reflux, which my GP treated with Prilosec.
The fact that you walked in with pancreatic cancer and got told you have heartburn doesn't exactly argue for regular checkups.