Not a lawyer but generally it is as you say: "because the trials suck after a while cuz no one remembers anything". It's not fair to have a trial when the evidence is unreliable because of the flow of time.

Encouraging timely action is another factor. Generally people with real harms will file sooner than later, otherwise why wait?

It's also to grant peace of mind -- so people can stop worrying about potential litigation after some amount of time.