Because you have much more choice for VPN providers than for ISPs. And you can change VPN periodically, far more easily than changing ISP. Also, you can use nested chains of VPNs, much like Tor, to distribute trust. So adversaries must compromise multiple providers, quickly enough that logs will be available.

Edit: Also, you can pick VPN providers outside your adversary's sphere of influence. That's standard advice for users in China, for example.

I've lived places where my only ISP choice was Comcast. I trust them as little as the worst VPNs, and having a choice of VPN lets you choose one which is trustworthy and in a convenient jurisdiction. That matters some in the States (no NSL to Canada, for instance) and a great deal in China or other countries.

    > But now the VPN provider can
    > just track you

Find one based in a less offensive jurisdiction?

It's not. There's no way to verify the VPN provider is not keeping logs and tracking you.

Well, you compartmentalize in multiple VMs. Using different VPNs, Tor, and nested chains of them.