And there's of course the second shoe that TouchWiz is a differentiator for Samsung (as far as they're concerned anyway, users may or may not agree): it's part of the "value" they provide, and it's (tentatively) part of keeping users on Samsung phones: Samsung also has TouchWiz skins not just for Android but for Bada (their in-home OS), for WiMo (Omnia 1 and 2, long dead) and older smart-ish phones.

So from a business perspective, Samsung considers TouchWiz important for phones released under the Samsung brand, they're not going to switch to stock Android UI.