Question

Why are worn out tyres discarded?

Answer 1

1. As tyre gets worn out the dynamic friction between the tyres and the road surface reduces thus during braking action the car is likely to slide. 
2. Worn out tyres is likely to burst due to any sharp object and thus forcing the wheels to move on Rim. Under such condition if brake is applied than the car can go under a spin.

Answer 2

As the tyre gets worn out the dynamic friction between the tyres and the road surface gets reduced, thus during braking action the car is likely to slide. Also worn out tyres are likely to burst due to any sharp object and thus forcing the wheels to move on rim. Under such conditions, if brake is applied then the car can go under a spin.