Because we build a false image of what we think God is and that is idolatry.
How far are we to go with that principle?
We would both agree that Shinto and Hindu are so far away as to be excluded.
Islam would be easily excludable by most people on Monachos.
Judaism--well, there might be some wavering.
How about Mormons?
Methodists, Anglicans, Presbyterians?
At what point do we draw the line and say "This group worships God, however incorrectly, but that group worships some other being."?
Why do we speak so certainly about some groups and not about others? Is not the actual divide between those who have accepted the guidance of the Church and everyone else who has not? Those who have not are in darkness, forced to fumble along as best they can on their own. Whether they have been blindfolded by others or choose to close their eyes, they are still in the dark. Thus, what does it profit us, as Christians, to argue over who is ten feet from a flashlight or 100 miles from a flashlight? In either case, is not someone still in the dark, without a flashlight, bound by a blindfold or closing their own eyes? Perhaps the man closer to the flashlight might be more likely to accidentally stumble upon it, but then he'd still have to teach himself how to turn it on, remember to open his eyes, or even more sadly, realize that he has been blindfolded so he will understand that it has to be taken off for him to see.
What does it matter if someone is blindfolded by a familiar bandanna or an alien keffiyeh?