Brake discs down after 26,000km???

Moin, I drive a 2017 convertible, 90 hp. When changing the winter tyres, I noticed that the brake discs are completely worn out. How can this be after 26,000 km? I feel hummed, because I bought the car with 20500km and there was a workshop service just before the purchase. The seller told me: “If you pick up your smart from us, then you don’t need the next 20,000 km to service”. Now I drove 5500 km in 10 months and already have to brake new 🙁