It's not just about storage. It's about fairness and creating an environment where people can transact in a relatively stable and predictable manner.

It is inherently unfair and destabilizing if people can sit on alleged wrongs, violations of agreements, etc. indefinitely, either intentionally or because they were ignorant or negligent, and then raise those wrongs in the court system any time they choose to.

A statute of limitations basically says "take your rights and obligations seriously and when you have a dispute over them with another party, raise it in a reasonable amount of time or forever hold your peace".

Even in the era of digital records it has a monetary cost; keeping decades of emails, documents, audio and video recordings is not free. I have friends working for software businesses that make many millions per customer just managing and retaining documents.