"they change often" is arguably a good reason for not keeping them. What advantage do you get from knowing that 10 years ago $IP was sending you spam if it's been though 20 different re-allocations and tens of thousands of 'actual owners' since then?
Imagine if google or cloudflare were logging every since query to their public DNS and correlating it with other access logs or google analytics or whatever. They'd be able to relatively trivially deanonymise huge numbers of actual people's identities and browsing history (beyond what they can obtain already).