Because there are things God wants to do through us. I’m not judging the Catholic church for mediating the relationship with God; I’m simply observing it. You can’t honestly argue that there isn’t some institutional mediation going on there.
Then every church mediates its members' relationship with God, not just the Catholics, given that definition. Every church has a leadership. The older the church the more established and hierarchical it becomes. And every time someone disagrees with that leadership there seems to be a point when either they are forced out or leave. That is why there "has" to be 1,000 flavors of Christianity, i.e., so many different denominations; often broken down to one single group or church building. It really is a shame, arguing about every meaningless nuance between grown men who are saved followers of Christ the Redeemer.
I think Jesus weeps every time someone starts a new Church because they couldn't abide the old one, rather than stay and work through the disagreements.
“Then every church mediates its members’ relationship with God, not just the Catholics, given that definition.”
I didn’t give a definition, but I do agree there’s always some kind of mediation going on. But it’s a question of degree, and as a matter of observation, it seems obvious to me that the Catholic church does more such mediation than Protestant churches do, just as a matter of doctrine.
As for Jesus weeping, note that I never condemned Catholicism for being what it is. I’m making an observation, and you (among others) are taking it as some sort of slight against Catholics.