Why do people say self publishing isn't as good as publishing a book with a company?

[old lady]

There's nothing wrong with publishing the book yourself, but everything wrong with trying to sell the book yourself. Most bookstores will not handle self-published books. So you have to find a way to let readers know about your book. That means advertising, and advertising is expensive. Even if you do it online, you still have to mail out the books to people who purchase them - and mailing is not inexpensive. In the long run, you are better off with a publishing company.
The only other option is ebook publishing - and you can self publish an ebook, but there again, the problem is with marketing. How do you let the reading public know about your book? You can tag all your facebook and twitter contacts, but as the returns on an ebook are very low, they aren't going to add up to very substantial amounts.