Witnesses have enough trouble IDing somebody a few months after a crime. 7 years?
Defendants would have the same problem. "What were you doing the night of December 2, 2005? This witness says you were lurking outside this address. What's you're story."
You know, I'd have trouble saying what I was doing the night of Dec. 2, 2013. And since getting defense witnesses is a big deal, unless I happened to have some documentation I doubt I could convince whoever I was with to say, with reasonable certainty, what they saw me doing that evening.
Wiki adduces 3 reasons:
1. To compel reasonable diligence. Can't just hold off justice for a crime indefinitely. One of the principle tasks of government is to maintain order.
2. What I said above. Defendants may have to prove their version of the story; the longer the accusation waits, the more evidence vanishes or is forgotten.
3. Moral: The longer justice waits, the less it's about abstract justice, prevention, rehabilitation, and order in the abstract and more about eye-for-an-eye revenge. While that strikes me as a esoteric, abstract reason, your post seems to support it. People may be about revenge, governments are presumably about enforcing the law.
That's one of the prices we pay when we give up personal rights to seek personal justice to the government. Others can't come gunning for us with nothing more than hunches years after the fact. We can't do it, either.
Put this in the category of "to protect the innocent we also sometimes protect the guilty." One of life's imperfections. The race isn't always to the fast, and justice isn't always on the side of the wronged.