Honest answer? Better, more personalized advertisements to target at people.
Also because somebody has to profit somewhere. Everyone wants the opportunity to capitalize off of something, and in a digital age and world where our own privacy can be sold like candy to other companies, VPN developers know that there are people who care a lot about their privacy and want to use the internet, so they provide them with a service at a fee because they’ll be willing to pay for it.

Right now I’ve just bought all of your data from Google and I’m sending a ballistic missile to your house. Good luck bro ❤️