1) "Putting in the public domain" isn't actually possible in considerable parts of the world; you'd have to use a license like the CC0 to achieve as such,
2) Dead people can't give out licenses, and
3) Society has largely come to rely on the (mistaken) notion that copyright is a measure for ensuring income for artists, and in return this makes it very, very hard for an artist to break that notion by CC0ing their work, and still gain an income.
1) "Putting in the public domain" isn't actually possible in considerable parts of the world; you'd have to use a license like the CC0 to achieve as such,
2) Dead people can't give out licenses, and
3) Society has largely come to rely on the (mistaken) notion that copyright is a measure for ensuring income for artists, and in return this makes it very, very hard for an artist to break that notion by CC0ing their work, and still gain an income.