Why are Elo ratings for go higher than for chess?
There is no ELO rating in go. And even no official international rating at all. A common question in go forums is "how does my rating in [whatever country or online server] compare with [other country or online server]".

The European Go Federation is maintaining an international rating system where Ke Jie is rated 2956.

goratings.org is an individual initiative by Remi Coulom. It has become popular because it gave journalists a way to assign a World ranking to Fan Hui, but this is not an official rating system.

Bottom line is your answer 3, there is some arbitrary difference in the way the approach is applied to go and chess.