WOW! Put yer thinkin' caps on.
First, weight for weight, a 7mm can get much higher BC's than a .30 cal. Meaning 180 VLD's, ELD-M's have higher BC's than the same 180 gr. weight bullet in .308. If you go with heavier bullets in a .308, you lose velocity.
Second, the reason it's shortened is because you need that extra little bit of room to seat super high BC bullets out farther where the ogive isn't down in the neck.
Third, the bullets need to be within a given OAL because if they aren't they won't fit current action lengths.
You and I could jump right to a long action, but can the military? No....they can't. They've built everything around one action length, 2.8".