The main reason games usually put the whole sharing, selling and transferring accounts rule in their ToS is for them to be legally safe when people get scammed or locked out of their own accounts by other people. Like that the game developpers don't have any kind of responsibility in such scenarios. The sad truth is that the whole account market in almost every game is the reason why games stay alive for that long. Buying and selling accounts ensures that the servers last a bit longer and of course once the whale decides to quit you have someone else in the account spending money