My first reason goes back to the time when a guy named David Marmon hired me to be his graduate assistant. I remember my first day on the job; I wanted to get my workout in. Like a typical meathead, I went out and did some crunches, bench press, and bicep curls. After my "gettin' swole" workout, I sat back down in the office expecting some praise and admiration for my superior workout skills, he greets me with "Why do you train that way?" I was dumbfounded. I thought I had all the answers when it came to things like getting stronger, fitter, leaner, and all around being more awesome. He followed that with, "We train movements, not muscles." I guess the idea of training movements, not muscles kind of stuck with me (even though we still did situps in our programming at the time). Train movements not muscles.
So, taking the train movement not muscles approach, is bringing your sternum closer to your pelvis a movement you want to get better at? Think about it. When you squat, what are the coaching cues your hear? Chest out. Back flat. Hips back. Right? Same thing with deadlifts and many other exercises. Chest out. Back flat. The spine evolved to handle weight in the chest out, hips back position. That's why we coach it that way. Why would we want to get better in producing force in any other way?
We don't coach people to get better at being in the rounded back position. So, why do they need to get better at it?
Secondly, the risk for injury when you flex the spine is high. Dr. Stuart McGill has become famous for saying, "Wanna see a disc explode? Keep flexing at the spine." The torso musculature (rectus abdominus, transverse abdominus, internal/external obliques, multifidus, diaphram, and pelvic floor muscles) was meant to transmit force, not produce it.
So, we train the limb muscles (ankles, knees, hips, shoulders, elbows, wrists) to produce force. we train the torso musculature to act as a conduit and transmit force, not produce it. Therefore, the stronger and more rigid we can make the conduit, the more force the body as a whole will be able to produce.