Why do some privacy friendly sites block access through VPNs?

I understand Facebook throwing a fit when I try to log in through a VPN, or some other mass surveillance company. But why does Proton throw captchas at me? I’m using their own VPN. Why does Libretube just not work at all if I’m connected to Proton VPN? Every time I want to access it, I have to turn off my VPN, which sort of defeats the point. I can enable Tailscale for custom DNS filtering, and it works fine, but I prefer having the full Proton VPN running 24/7.

Shouldn’t privacy focused tools have ways to work together rather than bashing heads like this?