Because the bias isn't as obvious. If one of the media majors is pushing for something political that is known and sorta expected. Anyone who cares knows that your choice of media source is potentially politically motivated.

When I browse through a blog post that speaks positively of a product, I know there is a pretty decent (double-digit percentages) chance that it is a well disguised paid ad. I can at least try to factor that in to my decision making.

Now with Google there is very much an assumption that they have an algorithm that applies some linear or non-linear math and comes up with a rating, sans some by-hand filtering for pornography and some other stuff that should be non-controversial to 80% of the population. If that assumption is wrong, government absolutely has a role to step in and expose that.