Because Forza Horizon 2 is highly sought after, yet difficult to buy digitally. Scammers prey on this desperation. Do not fall for it.
Even on Xbox, the game was officially delisted from the Microsoft Store in 2018 due to expiring licenses. Official digital keys are now nearly impossible to find. How to Actually Play Forza Horizon 2 on PC forza horizon 2 license keytxt pc link
If you want a native Forza experience on PC, you should look at the official releases available on the Microsoft Store or Steam : Because Forza Horizon 2 is highly sought after,