Also health reasons have the advantage that it is seen as somewhat rude to ask for details.
The other unspecific reason that is used similarly is "familiy issues"
While i can understand why someone would use this "excuse" i see it as a generally bad thing because it fosters that "health" and "familiy" issues are seen as excuses and not always taken seriously.
The fact that we are discussing that here kind of proofs this point.
“Family Reasons” And “Shorter commute” (to some other job)
The commute one I always thought was funny. It seemed to trigger universal empathy.
FTFY. 'Health reasons' are a subject that companies typically won't probe into because of existing legislation around Employers knowing employee's health information (And/or health discrimination lawsuit exposure).
'Family Issues' OTOH is something that's rude to ask about but far less likely to be problematic to actually go into. Also something that I probably -would- ask about in an interview because I'd like to know whether they could come up again.