Flash Gordon on the DVR now. Flash says, ‘Why aren’t I going with them?’
Is that acceptable? It sounds weird. I’d say, ‘Why am I not going with them?’ I suppose one could use ‘amn’t’, but that sounds a little archaic.
I will wait for the grammar experts to show up but it sounds fine to me.
An alternative would be:
‘Why ain’t I going with them?’ but that does sound wrong. Ain’t is in the dictionary for those that wonder BTW.
We really don’t have a good contraction for “am not”. The closest we do have is “ain’t”, which was more acceptable in early modern English than it is today.
I read in one dictionary that “aren’t I” sounded “womanish”, which I don’t get at all.
I’m something of a purist but I think “aren’t I” sounds fine in everyday speech. I would not use it in a formal speech or document.
It’s kind of just a little sub-note in English;
“Aren’t as a contraction of “am not” is used only in the question aren’t I? (= am I not?). In the non-interrogative form, the standard contraction of “I am not” is as “I’m not”.”
or…
“For which reasons are I not going with them?”
Wikipedia has an article on it
Funny I never though of it before
No normal human being ever uses “Why am I not going with them?” in everyday speech. “Why aren’t I” is the casual version in English. And since when does anybody look to formal correctness in dialog?
I’m a fairly normal human being, and this is how I would phrase it. The other way sound strange to me.
Not normal enough, I guess. 
Seriously, either you talk an abnormally formal dialect or you don’t pay close attention to your everyday casual speech. Since almost nobody does, except for linguists and writers, the latter is more likely.
“Why am I not going with them” would be an extremely rare location for normal conversation and is just impossible for casual movie speech. The only reason a writer would use that version is to tag the character with an unusual or overly-formal speech pattern.
Actually I’ve had friends comment on my syntax, occasionally saying it sounds too formal.
I also disagree. If I ever hear someone use “aren’t I…”, I feel terrible for them. The former is much more commonly used in my neck of the woods…“neck of the woods” is not.
I’ve personally never heard the phrase except as an elliptical interrogative, such as “I am right, aren’t I?”
It still sounds to me like fingernails on a chalkboard, regardless of how it’s used and its level of acceptedness.
“Why am I not” sounds perfectly normal to me, especially when followed by “surprised?”
Although when I say it, it often turns into “Why’m I not”.
Like Exapno Mapcase, I’m willing to bet you say “Aren’t I…?” all the time and you’re just not paying attention. If you don’t, I’m sure most of your friends do. Keep your ears open and you’ll hear it a lot.
I, myself, probably use it all the time. OTOH, the specific example does sound a little marked to me. I can’t really nail it down. Maybe you should ask the guys at Language Log.
I’d say something like “Why am I not…” too. The “Why” and “am” merge into one word, sounding like “Why’m”.
“Why aren’t I” sounds weird, to my ears.
If enough people used “ain’t” in everyday speech, wouldn’t it eventually become acceptable to the “grammar police”?
Probably not.
Doesn’t seem to have worked for many other things. Plenty of people out there bizarrely peeved over constructions that have been standard since before their parents were conceived. [The business about “ain’t” is at least a little understandable compared to those, since there are so many dialects which truly don’t employ it, though getting upset over it would still be a bit like faulting the Spanish for not speaking French]
The grammar police have a habit of employing bizarre “rules” that have no standing as actual rules of the English language and then justifying them with absurd contortions. Double negatives and split infinitives, the prescriptions against which are based on fallacious mathematical logic and Latin grammar respectively, have been a(n almost?) universal part of our language for a long time, and that hasn’t blunted the prescriptivists’ rage a whole lot.
It’s much less confusing in Ireland - we would just say “Why amn’t I going with them?”
All the more reason to use “ain’t”. In this case, it’s an acronym - Am I Not. I ain’t shy about saying “ain’t”. 