You are absolutely right, I oversimplified, but that has the advantage of giving us plenty of other experimental bodies to compare with (i.e. the solar system). For what it's worth, this oversimplification gives a pretty good average temperature of 15 deg celcius on earth.

Albedo is an important factor, but there is actually very much uncertainty about human influence on earths albedo, and at any rate the effect seems to be pretty small. Humans' effect on CO2 on the other hand is extremely well-established and much greater, which is why it's logical to focus on it.