Because the second you sign into something you expose yourself. A VPN stops people from identifying you by your IP, but when you sign into something you are given a cookie that your browser will submit with each request that uniquely identifies you. This is how websites can tell if you are auto-logged in.

Let's say you login to Facebook and then go to a website like pornhub for example. There are tracking scripts on their website from Facebook that will read your Facebook login tokin in order to load a pixel from Facebook that is unique to that page letting Facebook know that you are browsing pornhub.

Now let's say you haven't logged in to anything related to your main accounts. You still have to be careful about your browsing habits. Let's say you set up an anonymous Twitter, be careful of the people you choose to follow as that can be used to identify your region and start to build a profile on you. Another problem is that your browser will have the exact same OS fingerprint scan as your legitimate accounts if you've ever logged in from the same browser. There are many ways that you can accidentally give yourself away by giving someone enough puzzle pieces to figure out it's you.

Because the second you sign into something you expose yourself. A VPN stops people from identifying you by your IP, but when you sign into something you are given a cookie that your browser will submit with each request that uniquely identifies you. This is how websites can tell if you are auto-logged in.

Let's say you login to Facebook and then go to a website like pornhub for example. There are tracking scripts on their website from Facebook that will read your Facebook login tokin in order to load a pixel from Facebook that is unique to that page letting Facebook know that you are browsing pornhub.