I would have thought there would be tape on the BMC where there are gaps in the brake areas to reduce drag by a fraction.
hypothesising:
Perhapse they tried it and it didn’t stick welle enough as the brakes moved, or wrinkled and made it worse. Also there might be rules stipulating that bits have to be seperate components?
On sailing boats some classes only measure the hull, so that lead to designers fitting odd shaped keels which completely covered up sections of the hull, but had to clear by a few mm. I’m fairly sure there’s asimilar rule for the forks on a TT bike that they can’t be completly faired into the frame.