A lot of dangerous assumptions being made here.
First is that flankers are interchangeable and fairly coherent class- they’re neither. Pharah, DF, and Sombra are all going to operate differently. Historically, Tracer and Genji occupied different slots on pro teams.
Second is that conditions are constant. This doesn’t hold at all once you consider tank lines and how they function with the various maps, sides, and points. Some are more amenable to flankers. and it is hardly a clean 50:50.
Third is that any overrepresentation of flankers is due to DPS balance. We all know that tank balance can swing DPS from F to B tier and back in an instant with zero changes to any DPS. There’s also the popularity factor.
Fourth is the assumption that flankers are a discrete class. They’re not. Some are more suited to longer flanks than others, but like, 80% of DPS will benefit from going on flanks with some degree of regularity.