Without that time limit, an entire black market of non-app-store software distribution could spring up, and Apple would not be able to protect little Bobby who wants an emulator from bad app actors.
The real limitation isn't "seven days", as that's not that annoying: it is that you can have at most three such apps. But frankly, that still doesn't provide much "protection" from bad apps. All it is doing is prevent all of the apps on your phone from being pirated (capping that at three), which bounds the damage from adding this feature.
