Well.. that and average iOS users can't easily side load either. They need to jailbreak and that, itself, is a difficult task.
You don't have to - you can sign IPAs with a developer certificate, but the app will only work for 7 days on a free account (1 year for a 'real' developper account). Having to resign every week is obviously a dealbreaker.
This sounds like one of the Viva la Robolution slogans.