Actually this is a matter of military priorities.
With respect to Short Range Air Defense (basically to protect a unit from a helicopter attack, drone attack, maybe a light airplane or missile)... the US Army and Marines had those units, and progressively demobilized them over the 2000s and 2010s to focus on Counter Terrorism / Counter Insurgency... kind of how like it turned Armored Brigades that used tanks into Infantry of Stryker (medium) brigades. It didn't need it at the time, so it retrained troops and swapped out equipment. And now here we are , and the US needs it again and it finds it is going to take years (~5) to get it back to where it was back around 2005. This is what @ringpriest was referring to. US troops trained to fight the Taliban aren't going to do very well against the Russians without significant retraining and rearming (which is exactly what is happening). In fact, the Ukrainian Army, which fought in Afghanistan as part of the coalition with the US, found itself entirely outmatched against the Russians, because its skill set was all wrong.
With respect to tactical use of drones... thats a classic Pentagon fail. The US has by far the best large theater-level drones in the world. The MQ-9. The RQ-170. The RQ-180. Global Hawk. China and Russia are years, even decades from parity with respect to anything like that. But pairing a small bike sized drone with self propelled artillery? Or training infantry units to use backpack-sized drones? The US has been talking about that for years. And to a degree experimenting. Russia and China have actually done it.
So how can Russia do it and the US not? Again, the classic Pentagon fail. Russia went with a "good enough", largely commercial off the shelf solution. The Pentagon wants a requirement-laden miracle drone that they can use for 20 years. Sometimes that approach is best. In this case, with something that is rapidly advancing every two years, it's lead to the US having a whole lot of nothing other than canceled plans. Hell the Army is seriously thinking about killing its next-generation tactical data network (WIN-T) in the 2018 budget that would be integral to any kind of squad level use of drones to enable data-sharing.
The US has serious procurement problems. Too many requirements. Too many competing interests. Hell right now one of the biggest procurement shitstorms going on is a fight over if the Boeing 707-derived E-8 battlefield management and command and control aircraft should be replaced with a 767-derived replacement for a Gulfstream business jet. Both with the same actual militarized package, just a different airplane. The Air Force likes the Gulfstream. Lots of people don't like that.