If you're honestly asking, the reason this isn't done is because the main source of revenue for these free to play games are the people who become addicted. ("w

If you're honestly asking, the reason this isn't done is because the main source of revenue for these free to play games are the people who become addicted. ("whales") They spend enormous amounts of money and if they had the option to just upgrade then this would cut the legs out from the main chunk of the revenue the game generates.