Covere this in another thread a while back but apart from not being a good idea, it costs money we don't have. As expceted teams like Utd or Madrid/Barca etc can command a fee as everyone wants to play them and make money from a full house so they are selective and charge for the privilege of turning up. I imagine the Sven factor also has something to do with it being possible.
Pre-season is not about winning or playing big games imo it should be a gradual build of skills employed against you, as you increase match sharpness and togetherness in preparation for the seson ahead. I know it din't ultimately work out well but I thiught last pre-season was pretty good.