Actually, this is not the case. Lewis Machine and Tool and Connecticut Machine and Tool (who also have their own AR brand - Stag) make a large number of parts for the AR market; but they are not the only makers.
If you are talking actual forges producing AR receivers, there is Alcoa, Harvey, Martin Marietta, Kaiser, Cerro Metal, and probably a few others I left out. If you are talking about places that partially machine their own parts, there are at least two dozen I can think of off the top of my head.
Finally, it is important to remember that while the same place that makes a part for Colt might also make the same part for Model 1 Sales, it doesn't necessarily mean that the parts were made to the same specifications or even using the same metals. There are a hundred places to cut corners on an AR15 and make your price point better - and 99% of your customers would never know because they won't ever shoot the rifle enough to notice the difference.
To use just one example, instead of selling you a forged 7075T6 aluminium receiver, I can sell you a "billet" receiver made out of cheaper 6061 Aluminium - and charge you extra because it is a "billet" receiver, which everyone knows is better, right?
Or another fun one, I can save myself about a few cents per part by using pot metal for the receiver endplate on a telestock and loctiting it on instead of using the actual specified metal and staking it. You won't ever know unless you try to stake it or change the telestock. On the off chance you do stake it and find out the metal is brittle enough to shatter, I just send you a new one gratis with a snappy gizmo included. Maybe I even blame "a bad heat treatment" or something similar. You rave about my customer service and I still saved money on the other 1,000 that nobody noticed.
