Because it's neither obvious nor globally true. Legal status varies around the world and from a technical perspective an IP address on its own doesn't usually identify a person unless you have other information — account data, correlated data from other sites, etc. — and things like NAT and public wifi make that necessary to reliably link activity.

I think it's important to talk about this issue – especially the importance of not storing it long-term — but from my perspective the real concern is the industry dedicated to linking and sharing your online activity. Without that an IP has little value and with it they can deanonymize most people without using IPs.