Despite the claim that the manufacturers would have met such a market if it xisted, it sure seems from the frustrated people I've read that they still haven't.
There are a few possible explanations for that.
One is that there is a market, but it's too small to be worth the investment in developing an entirely new rifle. If you only sell a couple thousand units per year to people who are burying them, a large manufacturer probably won't see it as worth their time (and the knock to their reputation that might come from making a cheap, "disposable" rifle).
Another possibility is that it's too difficult to meet the price point you'd need to meet to make a new rifle "buryable". The Kel-Tec SU-16 is the closest thing on the market right now to what you're talking about, and it's still $400+, despite liberal use of inexpensive construction techniques.
There might be thousands of Mosins and SKSes buried across America, but that's because at one time you could buy them for $100. Even $300 is a bit steep compared to that. Sure, your design would use AR-compatible mags and ammo, but that advantage is offset by it being an unproven design (unlike the decades of reliable service that the Mosin and SKS can boast) that's built so cheaply the manufacturer won't warrant it past 10,000 rounds.
I think there is a market for a relatively inexpensive semi-auto that uses AR mags and has a more traditional rifle layout (those Kel-Tecs seem to sell pretty well), but I don't think making a rifle specifically to be buried in a PVC pipe for the apocalypse is much of a sustainable business model.