Great question! As a statistician that will likely have to design a weight loss trial, I have been wondering about the exact same issues and I completely agree with you.
The only answer I could come up with was that sponsors were following the FDA “Guidance for Industry Developing Products for Weight Management” in which percent change from baseline is specifically mentioned as the preferred primary endpoint (see Section IV.B.3.). Would be great with some input from a statistician from Novo Nordisk or Eli Lilly though.