If you’re paying for any software on your phone (e.g., games or productivity apps) you’re still likely giving the manufacturer money.
If you use any apps on your phone (e.g., the web browser or email) they can still collect analytics on you and sell that data.
If you pay for subscriptions through any of those same apps, again more money going to them.
If you buy too old of phones you’ll be left without critical updates, either very soon or right away, that protect your privacy and personal/financial data.