Er, sorry to disagree but rod action does affect loop size, massively so in extreme examples. If you stopped the rod on your forward cast in the identical place with a tip action rod as opposed to a mid action rod, the tip action rod would have the smaller loop as the mid action rod would follow through more on the stop opening up the loop. Do the same with a through action rod and the difference will be obvious. Ultimately however it's the user who dictates the loop shape as most casters don't have good enough loop control to utilise the difference in rod action, so it's a moot point anyway.
As for why we don't all use tip action rods, it's because they're not so fishing friendly as mid action rods, less give so less shock absorbtion more snapped tippets etc etc. Plus, they're less user friendly so harder to cast for the average guy.
A good casting rod doesn't automatically make a good fishing rod.