Not every player in Spain has these ludicrously high clauses. As has been pointed out, the difference is that they're mandatory there, so often it's just a token figure to service that requirement while in practice leaving the club without any obligation to sell.
Whereas here, there is no such stipulation. Surely it's obvious therefore that to have one entered in a contract means there must be some practical purpose to doing so: there simply wouldn't be any point setting it at £200m or whatever. It'll always be a compromise between the interests of the two parties: the player's desire to be able to release himself from the contract, and the club's desire to get as much money as possible.
Whereas here, there is no such stipulation. Surely it's obvious therefore that to have one entered in a contract means there must be some practical purpose to doing so: there simply wouldn't be any point setting it at £200m or whatever. It'll always be a compromise between the interests of the two parties: the player's desire to be able to release himself from the contract, and the club's desire to get as much money as possible.