Pads look fine to me. Hard to tell for sure without a side view, but it looks like plenty of 'meat' left. My guess is the last owner, or some previous owner, ran the pads down metal to metal, and the rivets dug a groove. When they got around to changing them, they never turned or replaced the rotor. So the new pads wore to the surface that they were given. Are both inside and outside pads close in thickness? If one is metal and one has life left, you could have a calipers/brake hose problem. If they are equal, or close to it, you have a sticking piston or the calipers isn't sliding.