Why do teachers insist on making us choose a career in year 10?

They're always like "Decide on a career now so you can choose your subjects according to your career". And my math teacher asked me what I wanted to do when I left school, and I said I had no idea, and he said well you better hurry up and decide. I seriously don't know what I wanna do when I leave school! It's too stressful to think about, especially when we have all these assignments to do and preparing for our school certificate. How do they expect everyone to know what they're gonna do when they leave school in year 10!?