Buying music cd or movie is waste of money, but buing LFS is investment for software that is in development and that already does teach you many things about car handling, it is going to help new drivers to learn to use right speeds in right places faster and also if something happens on road those who are good in LFS do have basic idea what to do to avoid accidents unlike those who have never practised with simulator or in real life.
Sorry, but your parents seem to be sheeps, they should open eyes and look things from other perspective than what they have used to, so they could actually see what really is waste of money and what is not.
LFS is not yet again one stupid computer game, even if you consider it as a game it has quite long life compared to almost any other game, you can easily divide cost to 5 years or even more as that is minimum time LFS will be top class game and still it will be sim where you can learn some basics from car handling that will help you at some part of life.
Because of LFS I have avoided getting to accident IRL as I did know instantly how car would react and what options I would have to avoid that other car, waste of money?
So tell your parents all this and if they still like to say it is waste of money they can come and say it face to face if they really think that.