Peruse Netflix's catalog of DVDs, and you'll find just about any movie or TV series you want. Look for many of the same titles in Netflix's Internet video library, and prepare to be exasperated. It's a problem that Netflix is unlikely to solve anytime soon. Despite an expensive expansion, Netflix's online video selection remains inferior. Netflix offers more than 60,000 titles that can be viewed on Internet-connected devices, while its DVD-by-mail service has more than twice that — some 140,000. Competitive tensions and Netflix's own financial limitations account for the huge disparity. No matter how much Netflix Inc. is willing to pay for the rights, some online video remains off-limits. Major movie studios are refusing to license the rights to most of their latest movies at the same time they're released on DVDs. Premium cable channels such as HBO and Showtime also are withholding their most popular series...