Because he didn't remove activation lock, nor prepare the phone properly for you as a new user.
If the phone is activation locked, you need to contact whoever you bought it from, have them remove activation lock or refund your money and give the phone back to him.
I see sberman provided the correct link which explains how to remove Activation Lock. Contact the person you bought the phone from and have him do what the link requires, or if he can't do it, get your money back. If you can't reach him (happens all the time), recycle the phone appropriately as Apple will NOT remove activation lock on that phone and we can't help you do so either.
See this user tip I wrote about buying used iOS devices --> The All Too Common SAD Reality of Buying … - Apple Community