Well, lets picture something for a second, then you can tell me why you don't think manufacturers have created heads with straight exhaust profiles yet.
View the attached images:
See the shape of the valves?
Now see how they sit in a cross-sectional view:
The second image is representative of a typical valved cylinder head.
Given the design you're attempting to achieve, your valves would have to operate in a completely different manner, so as to prevent the loss of gasses back into the head/oil containing areas, and to prevent the presence of mechanical parts in the (very hot) exhaust stream.