+1 aveodude...
manufacturing cost and emission restrictions play a big roll as well. it's just not practical for a manufacturer to use a tubular turbo header.
tubular headers are also use not just because of the better flow characteristics but to allow different turbos, turbo configuration, external waste gates.
the only reason manufacturers use 8.5:1 compression is to get the best performance on the cheapest gas and that makes it near impossible for detonation.