Because everyone can do it. No matter if You are rich or poor. No matter if You are overweight/underweight, frail, small or tall. You can play no matter what.
Actually in football history there was many cases that ill/crancky people become biggest stars.
For example Garrincha. Brasilian player who had one leg shorter. He was actually a cripple. When he came to the first training whole team laugh at him. But when he get the ball he pass along entire team and score a goal. He is called one of greatest "jugadors" in history. He was perfect dribbler. He won world Cup twice. He turned his weakness into strenght.
Michel Platini who was very tiny and skinny that even his coaches and doctors forbid him to play. He was playing alone shooting to garage doors as to the normal goal. And later he became greatest striker in Europe and one of the most topping players. He won Euro at 84' and was top scorer of France National Team. Also France was third at Mexico World Cup in 86'.
There is many more cases like this. Even today. Just find more about all those players which You think they are glamorous superstars. It wasn't so easy to them. They weren't born in rich family in huge residence like some of You may think. Just read a biography Zlatan Ibrahimovic, Cristiano Ronaldo, Lionel Messi, Ronaldinho.
Also another important and probably most important thing that USA citizens may don't understand.
The biggest advantage of football above any other sport especially USA invented sports like baseball and american football is that Football is international. It goes to larger crowd.
Football is international language. You may have different religion or politics and can be different in every way but if You love football You don't need words to express that.
Also it's kinda war game. UEFA Euro / Copa America or World Cup fascinate people from different regions, different countries, different cultures, different time zones. And every nation feels that connection between those 11 boys fighting on the pitch who represents their commonwealth.
Furthermore Football is beautiful quick and smarty game. Where weaker poorer team always can win with the stronger one. Also on the top level it's very difficult and demanding. Like in no any other game players actually uses their legs/feets to move the ball around. They can use any other part of a body except arms/hands. It requires something more than brutal strenght and speed or your physiqual apperence. In other sports like rugby or american football you don't need that. If you are powerful and quick it's enough. Here You actually gotta train more. You gotta exercise with and without the ball. You gotta be swift/strong/tough and also smart/cunning plus ball can't disturb You in the game.
In Football you actually can do anything You want. If You are central midfielder You can direct a game how you want. If You are goally You can save your team in crisis situation. And if You are forward You can score a key goal at 93' extra time. That feeling is better than sex.
Also to all fellows who are saying that football today reminds them of theatre. Haha that's ridiculous. You gotta know that after every match for example in English Premier League most of players run in the game above 12km and some even 15km. Name another sport where effort and pain is on similar level. Also in football every even small tackle fracture tibia-bones of players. After every game the got contusions and bone cracks. Taking painkillers and use tones of ointment to bruises.
Football is something more than sport. It's unpredictable. It can make a hero from a total looser. It can bring tears or joy. It's simple and brilliant. You can play anywhere. No need for an expensive equipment. Just ball and provisional goals and field. Just go out and play...