Same reason DVDs suck compared to pirating, you are forced to watch trailers, company logos and sluggy menus.
This is absolutely untrue. If you bothered to take 5 minutes to queue up and rip the DVD, you wouldn't see any trailers whatsoever. It's not like you have to manually spin the disc yourself when creating a digital copy.
When it comes to TV, however, piracy is a vastly superior experience, you get timely releases (in the UK the TV shows are often available for download before they're even aired here), the adverts are chopped out and the result can be watched at your leisure.
Even the BBC can't get it right, iPlayer doesn't have anything but the latest episodes, and even then only for a couple of weeks. But I absolutely positively feel entitled to everything I might download of theirs, because I bloody well paid for it in the first place- and not through choice.
Content providers and consumers will be forever at odds, traditional broadcast networks need to die out and be replaced with content providers who own server farms and can afford to buy up and resell a TV series, served from their network, at a sane per-episode/per-season price.
Funny, Apple are expanding their server farms, and they've got a lot of money to spare. Perhaps a future, big, overhyped TV series will be bid on and bought by this now multimedia company and distributed entirely online to their growing base of iDevice owners. If any one single company could pull this off, it would be Apple.