That's from defining the size of the billet needed for machining (or mould size if casting).
Plain AR-15 will fit 45mm long cases; the AR-10 will accept 51mm ones--and there's perhaps a dozen different rounds that each of those will accept without changing the geometry of the lower (the registered, expensive, bit). And you don't even need to change the pin locations on the uppers.
Now, try and fit a 57mm case or a 70 or 74mm case in there. That means redrafting everything. Lower and upper alike.
You can get pistol calibers into an AR-15 lower as the case lengths are typically all less than the 45mm length, so, a magwell adapter solves that issue.