Death is the biggest consequence of D&D. Sure, getting raised circumvents this, but it makes death a PITA. A
TPK, though, is the ultimate consequence.
Take the consequence completely from death, and the game feels cheapened. "Success" is no longer measured by "we beat them", but "we were stubborn enough to keep fighting even though we died a bunch of times." Your PC is no longer worried about surviving if you know that no matter what you do, you'll just repeat it again.
(Yes I know, there are some gamers out there in EnWorld who have taken death out of the game, and still find it enjoyable and have tension, but I think I speak for the vast majority of players when I say the above.)
I also think that at least to me, a save point would be really disappointing in an RPG. I
like death, in that it lets me play a new character. Or in the case of a
TPK, it offers the potential to do something really different, to play with the story a different way. It opens up new opportunities, like seeing the consequences of your party failing, or adding an extra wrinkle.
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