Back in the day, I assembled literally dozens of AR15s for an FFL, mostly OlyArms kits. OlyArms is usually considered one of the "lower tier" makers these days. In all those rifles, I never had to fit a single part with the exception of the disconnectors I mentioned. Some would malfunction due to poor disco shape, firing on the pull and release. We replaced those with Colt branded replacements and had no issues after.
Can a maker produce a "bad" AR15 part that doesn't work without fitting or modification? Sure, no one's perfect. But it is the exception rather than the rule, and personally I would return it for a replacement if it was out of spec to the point it took more than a minute or two of fitting. I did run into this issue just a few months ago assembling a friend's new AR, he had purchased a cheap imported M4 style stock and the buffer tube wouldn't thread into the receiver more than a couple turns before stopping. He exchanged it for a US made one that screwed in with no issues. Could I have lapped them together? Sure, and then an hour later he would have a receiver with loose threads if he ever changed the buffer tube, and the anodizing would have been gone. Here at the auto shop I work at, we replace a fair number of axles on front wheel drive cars. Every brand we've tried, some come in with the splines smashed from getting knocked around in transit. Now, I could grab a file and redo those splines to make them fit, but why? The parts store simply sends out another one and I don't spend $30 worth of shop time fixing something that should be right in the first place.