jobel it's because I'm use them to actually replace variables that I need to be saved. Some of these values, e.g. total time played in milliseconds, are set every tick.
I guess I should change everything over to global variables, but weirdly enough I've never seen any performance problems. Apart from the GPU side of things, the game seems to perform quite well on a lot of machines.
Again, '2012 me' would have implemented it a very different way if he knew any better. But due to a lack of best practices documentation, he really didn't know what to do. 2012 me didn't even know what file I/O was!