US sports are very different from the way they are structured, in comparison to their European counterparts. The length of a season is a very important example. There are so many sports that are popular in America, but they each have small seasons. The NFL season only lasts 5 or 6 months and the same goes for basketball and baseball. On the other hand, European sports usually have seasons that last 8 or 9 months, stretching from August until May. This means that they can be followed all year long and there is an excitement that is always building. They even have World Cups or European Championships every summer, so very rarely is there a time when the sport is not on.
The reason is because American sports are focused on the collective. Americans love so many sports but cannot keep up with 5 seasons that happen for 9 months, so they divide the calendar so that each sport can get its time in the limelight. On the other hand, Europeans love soccer and rugby and each of these sports lasts the year round and that is the major difference.
Also, American sportsmen are taken from their college teams, while in Europe kids begin training professionally from the ages of 14 or 15 and can make their pro debuts well before their 18th birthday. American athletes who reach the NFL or NBA are usually 21 or 22 by that time. So they have spent 4 years where they could have been playing in the NBA or NFL at college and that is a major difference with Europe. In many ways, that is a positive for American sports as their athletes have a college degree to make something of themselves when they leave sports, while in Europe retired athletes always end up doing something related to their sport.