It's partially talent, because 16:30 is not that easy for most people.
But training is a big part of it. There are enough people in the USA that we should have a large enough population of people with distance running genetics to be extremely competitive internationally. Here's a though experiment: How many Japanese live in America? How many Japanese-American men are elite in distance running? How many Japanese in Japan are elite in distance running? Shouldn't we have at least a few good Japanese men at 10000m and marathon? I'm not aware of any standout Japanese-American distance runners in the last 30-40 years. If the Japanese-Americans who do turn out for distance running trained the same way that Japanese in Japan do, we'd probably have some more sub 2:10 marathoners.
I'm not sure, since courses are often measured incorrectly, but I was a 16:30 XC guy in high school. My teammate, who I'd been within a few seconds of as a junior, ran 15:20 for 3 miles as a senior at state, where I ran 16:00. I ran 16:39 on the track as a Sophomore when my 1600/3200 PRs were 4:42/10:11. As a senior, my PRs were 2:02/4:23 (I was moved down in distance). I ran about 25-30 miles a week, and never saw much improvement above the mile (got 3200m down to 9:50 early in Jr year). My teammate, who I was within seconds of at every distance just a year before, was primarily running the 3200m and did about 50 miles a week. He ran 4:24/9:09 and 15:10 on the track as a SR. Although I was faster at 800m and 1600m, he was way better at anything above. The previous year, when we were all running 50+mpw for the first few weeks of track, I was untouchable at any distance. As soon as I was under 40mpw, I had no endurance (but my 800m suddenly blossomed).
Another guy, who I never lost to at any distance, trained with us in the summer, doing 50mpw with my teammate since they were doing 5000m for Junior Olympics (I was still doing 25-30 and running the 1500m). He ran 15:45. I set my 5000m PR in college: 15:45. I was massively overtrained because as soon as I moved up to college, I tried to do 50-80mpw but blasting every run way faster than I ever ran in high school. Based on the guy I never lost to running 15:45, I'm pretty sure I could have run 15:40 in HS even with my low mileage, and probably no worse than 15:15 if I had been training for longer distances like my teammate (he went on to run 13:59 and another teammate I often beat ran 14:15 and was ranked top 10 in the USA for a few years in the marathon). The training makes a big difference.
Distance races like 5000m require a lot of aerobic development. We have to start our kids younger, not be afraid of giving them a lot of distance (obviously building it up sensibly), and most importantly we must make sure they run at an appropriate pace and don't overtrain. I would bet that most high school 16:30 guys are 1) not running enough distance, 2) running most of their aerobic development runs way too fast because they think the best way to get better is by trying to keep up with the fastest guy on the team instead of running the pace that will lead to greater development.
I thought my high school coach was good because the only competition I had was my teammates and we blew everyone else away. But we were all 16:30 XC guys, probably with talent to do much more. My one teammate became a HS 15:10 guy, my other teammate became a 48:00 10 miler and top US marathoner when he got his training right, and I went on to permanently suck because of undertraining in HS and overtraining in college. Imagine what kids were doing at the high schools we dominated on...lots of < 20 miles a week and no running in the off season (we ran in the summer but not in the winter LOL).