Here in Denmark Granny Smith is pimped as a regular apple for eating raw. When you say it's a baking apple, it makes more sense that the taste is so unappetizing when eaten raw.
Yeah, the balance of sweet and tart that you would expect from an eating apple is missing from Granny Smith, because that would be lost when you dump it in a pie crust with a cup of sugar. It also explains why the apple is so firm, because it's expected to soften when baked. You can eat it raw, I just don't know why you would.