Few authors sell enough books to make a living at it, and this is true whether they sell through publishers, Amazon, or on their own.

As with any business, how well they do with their own site is strongly dependent on:

1. the quality of the book

2. the promotion and marketing

3. customer service

4. price

None of this is trivial, but it is certainly doable. Small businesses thrive (and fail) all over the internet, selling books is hardly any different.

And, of course, the 100% failure rate happens only when you quit before you start.