I think there are two arguments here. Some argue 'why should they be paid so much for kicking a ball about' which taken in complete isolation from a major club, fans, the whole football industry makes sense. However, like an actor or actress, musician, artist their skill is something which is part of an international brand which brings money into the club in the form of merchandise, sponsorship deals, tickets for games, TV rights etc. This is no different really to Rihanna, Beyonce, Angelina Jolie, Cameron Diaz, Johnny Depp receiving a lot of money for using their talent in a way which brings money in to a record company or film studio.
Someone who plays football vs. other professions argument is a fair one. Should kicking a ball about be paid as much as a nurse, soldier etc?
A footballer playing for an well-known club in a top league and internationally should be entitled to more money. This is reflected by the discrepancies in the pay of those in lower leagues. It isn't about the fact they play football, it is what their skill can bring to the club in terms of revenue etc.