In the book Fast Food Nation, there’s a few page section where author Eric Schlosser talks about artificial flavors. He visits a lab where they create them, and meets with a ‘flavorist’. Creating artificial flavors is described as sort of a mix of science and art. He lists the ingredients in a typical ‘artificial strawberry flavoring’, and it’s about a 3-inch high paragraph of chemicals like benzyl isobutyrate, diacetyl, ethyl propionate, and a whole bunch of other things I’d rather not know about. These flavorists have fake-sounding but apparently real machinery like ‘gas chromatographs’, ‘spectrometers’, and ‘headspace vapor analyzers’ to help them pin down a foods flavorings. But even still, it’s an imperfect science. Who knows? Maybe the perfect bananna flavoring is the Holy Grail of these guys?
He doesn’t talk specifically about artifical bananna flavor, but there is one section that might be part of the reason you’re looking for. He talks about the importance of “mouthfeel” – textures and chemical interactions that effect how you percieve flavor. Banannas have such a distinct mouthfeel, that even if they get the flavoring reasonably close (not saying they have), it’s still never going to be quite right.