I've never heard of a veggie burger with chemical flavoring. Most of them aren't intended to simulate beef; they're a combination of various savory, protein-rich plants and some seasoning. I'm not a vegetarian, but sometimes I enjoy a bean patty anyway.

As for this new wave that is intended to simulate real meat, the flavor is completely natural, just found in an unusual source outside of actual meat:

> Impossible Foods' scientists discovered that heme is a key factor in how meat behaves.[15] Heme is the molecule that gives blood its red color and helps carry oxygen in living organisms.[16] Heme is abundant in animal muscle tissue and is also found naturally in all living organisms.[17] Plants, particularly nitrogen-fixing plants and legumes, also contain heme.[18] The plant-based heme molecule is identical to the heme molecule found in meat.[19][20]

> To produce heme protein from non-animal sources, Impossible Foods selected the leghemoglobin molecule found naturally in the roots of soy plants.[21] To make it in large quantities, the company's scientists genetically engineered a yeast and used a fermentation process very similar to the brewing process used to make some types of beer.

https://en.wikipedia.org/wiki/Impossible_Foods