Could someone please explain this football rule............?

Why is it that if a offense player has the ball and the clock expires, why is he still able to continue running towards the endzone for a touchdown even though the clock is already dead? Shouldnt it be that when the clock stops, the game or period is over regardless? Whats the point? And What would happen if during the kickoff at the beginning of the game, the ball travels 100 yards and goes through the goalpost? Has this ever happened and what if it did happen? Would it be a touchdown or field goal and would anyone score anything? Why is football the overwhelming most popular sport in United States outranking baseball, basketball, hockey and other sports by such a large margin? Why do so many people love the game that much? Which do you think is harder, scoring a goal in soccer or scoring a touchdown in football? Why does everyone think football is so much harder when scoring a goal is much much more hard? Why does a team only have to advance the ball 10 yards to get another set of downs? Moving the ball 10 yards is nothing. Shouldnt it be 4 downs for a touchdown? What are the names and terms of all the different plays? and why are they important? How many difference types of plays are there? What makes a game exciting or boring? and why. After a team scores a touchdown or field goal, do they get to keep the ball and keep playing defense or do they have to give the ball to the other team? How could a team win if they losing by a very large margin? Can a team score anything from defense? What percent of all games go into overtime? How rare is an overtime? Why arent teams allowed to have more than 11 players on the field at once? Why do they sometimes break the rule? How could they not get away with that with all the referees?