Well... there are only so many practical ways to build a rifle or handgun. The AK 47 dates back to, well. 1947 (earlier than that actually, it was adopted in 1947) and takes influence from several preexisting guns. The AR 15/10/M16 platform goes back to the early 1950's, and draws it's gas impingement system from early research just prior to WWII. John Moses Browning's various handgun systems have been more or less the final word since the 1930's, and what you get now are only variations on the them and outside package. Pretty much any modern small arm is going to be a refinement of earlier concepts, or clever new uses of modern materials or ideas (polymer frames, bullpup stocks, etc...). And, given the need to have a reliable, functioning weapon for military use, ideas that are tried, proven, and work tend to stick around and only get replaced by refinements or radical new ways of doing things.

At the end of the day though, there are only so many practical ways to tap the gas from a spent round and use it to cycle an action, and once you get into the heart of a weapon, you could go back as much as a century or more to find the original root tech.