American sports

As a european football fan who watches a lot of european football, i cant understand how american sports work. What is the point of playing in lower leagues if you never get promoted or relegated between leagues. Wouldnt it be completley pointless to play for a lower league team than for a team in the top leagues? Why play at all if you know you are a mediocre player who will never play in the Nfl, Mls, Nba and Mlb etc?