Why does canned food expire soon once the can is opened?
No.

When you can something, the temperature and pressure kill bacteria and micro-organisms. 

Once its exposed to air, you're exposing it to bacteria and micro-organisms. Despite you thinking the spoon is clean, it still has some bacteria and micro-organisms on it which will get into the food. And your "airtight" container is likely not nearly as airtight as a can.