Uh, LMT, Noveske, Colt certainly make entire rifles. I'm sitting here holding an LMT Defender 2000. FN does, but it's .mil only. Uppers and BCG only for BCM.
For the same price or just a smidgen more than Bushmaster, you can have one of the higher level brands. S&W, Charles Daly, LMT, a much smaller brand or two...
If you are going to be adding a rail later anyway, one of the best deals going is the Daniel Defense rifle for $1600. It has an excellent rail and a BUIS, plus a Mappul MOE stock.
The biggest problem these days is availability. Before the election, you could have an LMT complete for a grand, although you might have had to wait a little.
If you are looking for a serious use carbine, or appreciate quality worksmanship, then all AR-15s are certainly not equal, nor is it a "Ford vs. Chevy" kind of deal.
Now, you are talking about Olympic, DPMS, and a few others, then, yeah, they are pretty much alike. You can, however, alleviate some issues by doing corrective work such as gas key staking, bolt replacement, chamber reaming, etc.