Those underlying ISPs have data that can be used to correlate (with a scary amount of accuracy in a remarkably short space of time) your flow to the VPN server against a flow from the VPN server to a remote host.

Without exaggerating, lives have been saved through this technique. Once the technology exists for one purpose, it can be repurposed very easily.

Money

>Why would you assume that the majority of VPN providers lie to customers?

Money. There's no way to confirm their claims, and if logging+selling data= more money, there's a reasonable assumption that is indeed the case.

>Why do you think people who want to stay anonymous don't know how to throw off browser fingerprinting?

Even if they do, it's difficult, ever changing, and easy to slip up.