Here's several reasons:
1. Some folks need a super challenge, they're too good at the game.
2. Some also believe the simulation tires are the only ones that truly simulates the way a car handles.
3. If you can control yourself, simulation tires will give you the longest, most dramatic drifts. IF you can control yourself.
4. They're inexpensive.
5. They are supposed to simulate the tires found on everyday passenger cars. Normal tires are not really true-to-life according to what the manual says.
Personally, I never use sim tires....they take the fun out of the game for me. I prefer being relatively equal terms against other cars. I like a challenge but not THAT much of a challenge!