I was wondering while I was researching the p320, why doesn't sig sell the commando but they sell ar15's? Same goes for companies like H&K and so on. Why not sell their own proprietary guns like FN does? I can't personally see myself purchasing a sig ar when I can just build my own for cheaper and get more out of it. This becomes especially jarring in H&K's case cause the only way to get a 91 or any variation of it is through custom shops for thousands.
FN makes AR-15s for the govt and you can buy civilian type AR's from them as well.
First, Eugene Stoner's rifle was designed from the get go to be easily produced using new but developed industrial technology at the time. Gas impingement rifles are a bit cheaper to produce than piston based, the barrel headspacing using a barrel nut eliminated several production steps, using composites for the stock, etc. meant using molds rather than dealing with wood issues, and so forth.
Second, the economies of scale from the whole host of subcontractors that made AR parts has made the rifle cheaper than Sig, FN, or HK, or any other can produce their model. Note that Springfield Armory cannot make a M1A cheaper than an AR because the design is more complicated and more expensive to produce (compare the work required to machine a steel Garand or M1A receiver versus aluminum forgings used in the AR) and the company does not produce enough to get economies of scale relative to the AR. One good comparison is the AR-10 platform which uses the same technologies as the AR-15 which makes is potentially a bit cheaper to produce than the M1A, however, because there is not a unified platform nor significant demand, those making the AR 10 don't get the same economies of scale. Thus we see that AR15's are cheapest, AR10 quite a bit more, and then we get to other platforms. The economies of scale and ease of production has also made AR15's cheaper than Mini14's.
Thus, the marginal, if any improvements in ergonomics, function, accuracy, etc. for the average consumer of Sig, FN, Tavor, HK, Mini-14 platforms is outweighed by significantly higher costs of those platforms (double or more) without commensurate (doubling of utility from the "better" Sig, FN, Tavor, etc. platforms) for the average consumer. Simply put, the average consumer does not perceive the value from the alternative platforms as worth their current price--hence they are niche products and unlikely to break the pattern without some large consumer of military rifles such as the U.S. Military, etc. adopting them and/or new manufacturing technologies that alter the mfg cost equation. AR's are simply good enough for most people which means that makers of firearms either resign themselves to niche products at higher prices or play in the AR market where competitive forces constrain their prices.
As usual, YMMV.