We never used to have to brush our kitties, and never had a problem until we got Wylie (M. Coon, 1 yr. old).
He plays in water all the time, so his whole underside and around his neck got badly matted.
We TRIED to cut some of them out, but they were so close to the skin and would not brush out either.
We finally had to take him in to be shaved...............not NAKED like Spencer, but quite a bit was taken off the underside & neck area.
He looks scruffy now, but it will grow out eventually. We have been trying to brush him a bit every few days so that he gets used to it and does not squirm...........he's getting better.