Remember also, that there simply were not anywhere near as many guns in America in 1770 as there were in, say, 1830. Not many survived because there weren't all that many to begin with.
And, there are tons of "plain" guns that still exist...from the 19th century, but that was the style at the time.
We went through a major change in popular decorative arts during this period. From Baroque in the early 18th century, through Rococco in the middle of the 18th century to the (ugh) NeoClassical in the late 18th/early 19th centuries. Not just with guns, but with everything else. Carving disappeared from guns, simply because that was no longer in style. Instead of carving, you get lots of inlays, often well beyond the point of being gaudy. Also, gunmaking became more and more a factory industry. Gunmakers were spending less time doing hand work or decorative work. The demand for guns was also increasing greatly, as population grew. The gunmakers didn't have time to put much decoration on their guns.
In the 18th century, I believe even the "plainest" of guns would have some carving (and the extant ones do), simply because that was just the way it was done. It was expected, and doing moulding lines, teardrops, a leafy thing behind the tang, etc, doesn't take an awful lot of time to do, really, so wouldn't increase the cost of the gun appreciably.