Could be that the aliens have found a good way to hide, and they are just waiting for us to annihilate ourselves, after which they will take over what is left. The increase in activity in the last several decades could indicate that they see our end as nigh.

The problem for the aliens is that, knowing our race, there won't be anything left to inherit after we're gone. We will have turned Earth into another Mars.

So they had better reconsider intervening.