http://techcrunch.com/2012/01/25/netflixs-streaming-dvd-marg...
(That's old, they've dropped precipitously since then to something like 48%...)
DVD rental is a high-margin but stagnating market, while streaming is a low-margin but growing market.
The stock market believes that it's better to expand profitlessly than to remain small but profitable. See Amazon's P/E ratio.
Besides, Reed Hastings is a tech guy. He cut his teeth on the Purify memory leak detector. He claims that he always wanted to do streaming -- it's just that the broadband infrastructure wasn't well-developed In 1997, whereas DVDs were just then starting to take off. (And they happened to weigh under an ounce, which made them cheaply mailable.)
Some people think that being a tech guy is exactly what's blinding Reed to the fact that the licensing problem is a far bigger problem than the shipping costs..