The rider is a much bigger factor, of course. But that’s not the problem being addressed here. You’re not suggesting the bike should not be optimized are you?
It makes no sense to optimize the bike in isolation. There is complex airflow interaction between the bike and rider. There are changes that aerodynamicists could do to reduce bike drag that would actually increase drag on the complete bike plus rider system.
The problem is that there are so many variables with riders. For example, just a slight change in head position can increase drag more than any improvement in frame tube shape could save.
The problem is that there are so many variables with riders. For example, just a slight change in head position can increase drag more than any improvement in frame tube shape could save.