I am really surprised this didn't kick up

I'm sure corporate, or at least the engineering dept. knew about chassis flex. It's one thing to know about it, but it's an other to deal with it. No computers in those days, everything was computed with a slide rule and the engineering time and cost to do those calculations was extreme. It is strictly a dollar function. I believe that is why frames were replaced with unibodies and why such things as full perimeter unibody frames were omitted. And what everyone has said on this post is correct, the bean counters ruled the roost. Run it on an app? Nope didn't exist.
Back "in the day" I had a 1950 Olds with a 57 J2 engine. Talk about chassis torque! first thing that would happen on launch is the back end (factory coil springs with truck arms) would go skyward until the shocks reached full upward travel, then the fun stuff would happen, some of you "senior' folks may remember that. You could judge how good your launch was by the front end twist. And yes, you could see and feel it. Dick Landy's '65 Dodge used to launch that way too.