road bike - how to choose chainring (crankset) and cassette size?
The main reasons your combination is avoided are as follows:


It would require a long cage derailleur. The maximum a Shimano short cage derailleur can handle is 33 teeth, and your combination would require 37 teeth. Long cage means extra weight, wind resistance and cost, none of which the manufacturer wants.
34/28 is already extremely low. Not many people who ride racing bikes need gears lower than that. Compare that until few years ago, the standard used to be 52/39 and 12-23.


Of course, since you are building a custom bike, you can just order a long cage derailleur and build what you want.