It's because they don't want the liability and potential costs of investigation. You're right, it could be stored indefinitely. It's likely not worth keeping all records after 10 or 20 years vs the continual cost of storing it. But certainly longer than 30 days.

I used to work in public safety, 911 systems primarily.

Chicago holds 911 and radio recordings for 90 days.

They could hold them longer with the equipment they have. They don't want to. They want to be able to respond to all information requests with the fact that they only retain data for 90 days.