Why do doctors usually induce early?

They shouldn't unless you are high risk in some way! If nothing is going on where you need to be induced, you should definitely advocate to be induced later! It's much more likely that you will have a better labor :)

Only if medically necessary

High risk, preeclampsia, something wrong with baby or you, low fluids

They should for medical reasons only, however the do it mainly for convenience. In my opinion.