Originally Posted by
Milo's Ghost
Two reasons (since the above explanations apparently didn't sink in):
1. It shortens the ROM. It's like cutting your squat above parallel. No bueno.
2. It's not consistently reproducible. If you allow yourself to start doing it, as the weight gets heavier, you'll bridge a little more each time. So if your bench goes from 175-205, did you actually get stronger? Or is your bridge just higher?
Now, I know what you're going to say (because I'm a ghost): 'But doesn't arching/keeping the chest up also shorten the ROM??????' Well, yes. A bit, tho not nearly as much as bridging. But there are reasons the line is drawn where it is. The arch is reproducible in a way that the bridge is not. Also, the arch serves many other useful purposes - it keeps the back tight, engages the lats and hips to help keep the trunk solid, and allows the legs to help brace the torso as well. When you arch, you have a much more stable platform to press against. As in all things lifting, tightness and stability are good; looseness is bad.
If none of the above explanations (from myself or others) is 'coherent' to you, then lifting is probably the least of your worries. You may not agree - what with your decades of lifting experience and all - but it is what it is.