Good question! I never really asked myself "why!" There IS liason with aller, avoir but not with être; perhaps because it could be confused with "tête?" I'll see if I can find a "rule" for that!
Mme
Why is there a liaison with "comment allez-vous" and not with "comment est-il"?
2 answers
P.S. Although I did find a rule about after "adverbs" (comment...) it did not seem to be crystal clear. For that reason, I asked l'Académie Française and am awaiting clarification.
Mme
Mme