Banana Synthesis of Ester | Ester | Alcohol
My belief, which I have no sources help support, is that many decades ago when they were first developing artificial flavor they noticed that isoamyl acetate was able to be close enough to the Gros Michel, the common banana of the time, that it was recognizable as banana. It may not have been a perfect match, but was close enough to commercialize as artificial banan flavoring. When the Gros Michel was not mass producable anymore and the Cavendish became common in middle of the 20th century, the industry just didn't change the artificial flavor. Whether it was due to the isoamyl acetate being so much easier/cheaper to manufacture than a more accurate simulation of the cavendish or if it was just that the consumers came to expect a specific taste and there was no money in improving it, or some other reason, nobody knows for sure.
Synthesis of Ester- Isoamyl ACETATE (BANANA FLAVOR) ..
Synthesis of Fragrance Esters - YouTube
Also present in apple, apricot, banana, papaya, wheat bread, other breads, cheddar cheese, Swiss cheese, coffee, black tea, roasted filbert, plum brandy and cooked shrimp.
Synthesis of Fragrant Esters Lab Part I & II - …
There are a group of chemicals called Esters that are very common in plants and it can have a big impact on their taste/smell.  Some fruit/vegetables have greater concentrations of one or more of them. One of them, isoamyl acetate, is the primary flavoring in artificial banana flavor. I believe esters are relatively easy/cheap to manufacture.