Seriously, as fietsbob mentioned, the amount of perch on the ramp portion of the bars is compromised by the need for a sweeping top curve for wrist clearance.
Also the width of grip at the top of the bar is narrowed by having gently-sweeping top curves, so having the bottom part flared outward a bit allows the rider to have what some would consider the best of both worlds, a wide upper grip and long, level ramps, still with no wrist contact at the top "corners".

I wanted a "full-curve" i.e. 180-degree loop bar on this not-so-large frame, for some added reach at the hoods and for a higher hood position, so I replaced the old French bars with this Trek System-3 handlebar in a wider width. I had wrist-burn while climbing though, but instead of replacing the bars again I used a 4-foot long piece of inch-thick Trex decking to pry out the lowers just 3/8" on each side, enough to eliminate the wrist contact.
It worked well because I didn't need or want the "rise" of a traditional Rondonneur bar, just the full curve, some added width, and enough wrist clearance. Shown here, installed, before I pried the lowers out. I did the prying with bike laying on it's side, which went smoothly.
The original French bars/stem is shown on the ground, had an entirely different feel which was far from the best for aggressive riding.