I don't think it's all about marketing. When I was in school, I took care to look my best when facing a difficult test. Other students did the same. We felt more confident when we felt we looked good. Your footballers may feel the same.
I don't appreciate the modern young person's approach to hair. For example, I just studied a lot of pix of actresses at the Academy awards show. Their dresses were fancy, but their hair looked bad - seemed dirty or sloppy. Most of them looked like they had pulled their hair back to get it out of way while they turned the compost. This unkempt look is very common.
The American footballers referred to might have been adopting that look.
A few months ago I sent my ISP an e-mail saying that if they were going to post a picture on my screen and force me to look at it again and again, then the least they could do is ensure that the models don't have dirty hair.
So, MtheGM, I congratulate on how your footballers look.