I agree with Vatdim. Curvy tubes are the output of designers, not engineers.

As the large companies push the envelope with frame weight, the curvy tubes are disappearing. Why?

Because a straight tube is stiffer and stronger.

It's easy to demonstrate: make a straight tube and a curved tube the same length and with the same layup and compress them axially to fracture. The curved tube will show more strain for a given load (eg it's less stiff) and will collapse at a much lower load. This was predicted by Leonhard Euler almost three centuries ago (Google Euler column buckling).

The same goes with extensional or torsional loads, for different reasons.

An arch is strong when loaded on the arch, which a bicycle tube isn't.