Uh, a studio and a concert hall have totally different requirements. Why are headlight reflectors parabolic rather than cubic? Why isn't the mirror replaced with black non-reflective color so that you can see the bulb more clearly?
Now if you go "what?" you have my reaction when reading your question. The whole point of a concert hall is to provide the acoustics for letting the sound reach the listener in sufficient loudness and with the plasticity of a large room's acoustics.
In contrast, a recording studio wants to have as little acoustics as possible, with only direct sound reaching the microphone. That makes it possible to work out coherent acoustics for combining all of the successive takes: all of the placement and reverberation and room acoustics is consistent and "just right" because it has been added post-production.
But adding those post-production is not an option in live venues. You want to make the most of the sound using mechanical and acoustical means. Cubic stages do not project as well as round ones, and they have awfully pronounced "modes" of resonance resulting in uneven loudness at different frequencies. And anechoic layers in excess would swallow the sound of the artist, resulting in tinny dry sound that is far too quiet.