A lot of great posts here. I see it as a 3-fold problem. I've been known to talk nonsense before so take it with a grain of salt.
1) The US is in decent shape. Pulisic, McKinnie, Weah and a host of other young kids are doing very well. The focus of the national team seems to have moved to the younger generation, which should be good. These guys could potentially be a part of 3 world cups. The MLS is making strides, there are a ton of good young players and I think things are moving in a positive direction. BUT, as others have pointed out, many other countries are doing this too. Many countries want to succeed at the world cup and have the support of their entire county in that goal. By and large, the US missing the WC was a funny twitter meme. Yeah, some people were angry and upset, but the levels of angst compared to Italy were far different.
2) Few options for players to stay home, learn the game, and turn into an elite player. As has been mentioned in this thread, its' difficult to go overseas. If you're a kid in England, you can find a great academy and stay at home or your family can move with fewer issues. But moving overseas? That's hard. England has a pyramid system of clubs that have academies where kids get proper coaching. This, however, may be changing. Alphonso Davies is 17 and has been trained for Vancouver (MLS) and is being sought after by many teams in Europe. (he's Canadian, not American btw). The key is to get young kids quality training and instruction so that they can get to the MLS and get discovered by European teams. I think with the MLS academies, this will begin to happen more and more. If the youth system were bare, this would be a huge concern. But it's not. It appears things are going well and progressing.
3) Talent. I don't fault people for saying that "we gotta get the best athletes to play the sport". It makes sense and there are certainly arguments to be made for doing that. The only problem is that the "athletic skills" needed for soccer don't really translate to other sports as well. It's not as if our NBA players would be world cup stars if they had only begun life with a soccer ball at their feet. Lebron James would not necessarily make a superior soccer player. BUT...I think there's still some truth in this. The soccer players players produced by the US are rarely elite athletes. France has a superstar 19-year old Kylian Mbappe. He truly has next level speed.
https://www.si.com/soccer/2017/12/11/kylian-mbappe-records-frightening-speed-statistics-paris-saint-germains-win-over-lille
His top levels have approached Usain Bolt levels of speed. The US has never to my knowledge, had an athlete like him in the soccer team. BUT....the US has plenty of athletes at his level or higher playing other sports. If Mbappe's parents had lived in the US, he would never have made the soccer team. He would have been stolen by the "bigger sports" early in life. So while the athletes that the US has on the soccer team are plenty athletic enough to compete in the WC, it cannot be denied that the upper echelon of athletes in the USA don't focus on soccer from an early age. I'm shocked to see some in this thread try to deny that or act as if this is a "newbie" argument. In that I wholeheartedly disagree. Obviously being fast doesn't equal success in soccer, but the US pushes out elite athletes year after year, and few of them decide to play soccer full-time.
In any event, I think things are slowly improving in this country. The MLS is the quickest path to change, so if this means something to you, you should go support your local MLS team.