What explains the hair and fashion focus of many European football stars vs. pretty much a complete absence of this among US star pro athletes?

The European soccer stars are international brands that get paid millions of dollars/pounds/euros to sell product. It's in their interests big time to develop and cultivate their brand, which includes looking like rock stars. The American soccer players, quite simply, are not. If your question is about American athletes in general, I would disagree with your premise.

The US national team is pretty stylish this time. A few of the players have long hair. Graham Zusi, Mikkel Diskerud, DeAndre Yedlin and Kyle Beckermen seem to have a hairstyle comparable to European stars. There are always narcissistic players in every national team. Beckham for England, Ronaldo for Portugal, Ramos for Spain. Compared to US football stars tend not to be as popular as footballers in Europe. So much focus on players makes them feel the need to be well groomed. Personally, I feel that the Dutch national team at this World Cup is the least stylish. Only Daley Blind has long hair amongst the starting lineup. I also agree with the premise that Kirti and Jeffrey have put forth. Thanks for the expensive A2A, Anon.

I tend not to agree with the implication that US pro stars are not glamorous per se. Host of names like Muhammad Ali, Agassi, William sisters, Shaq-o-Nile come to mind. A cursory search on Google will tell you that European soccer is the most watched sports in the world. That would surely attract more big brands to it and get them to sign up stars sportsman as endorsers. Some of them groom well as part of these deals. This may be a reason why it may come across to you the way it does.

Two words: Brek Shea (Stoke City and US National Team):