ISPs get legal notices from companies and are liable if it is found that their users are downloading illegal torrents and they don’t take action against those users.
How are VPNs any different? By using a VPN, aren’t you essentially transferring your accountability to the VPN provider? Wouldn’t courts find that since this or that VPN service’s exit server was used in ____ illegal online activity, they’re responsible and must cease operations?
How do VPNs operate? Are laws different for them? If yes, then how does that benefit the state? Wouldn’t the state benefit from treating VPNs the same as ISPs so they get more control?
The scenario you describe with ISPs is pretty US-centric, as are the various copyright laws and companies backing it, which is (one of the reasons) why many of the most successful VPN companies are either not based in the US (and most have server nodes that are not too).
Mullvad is from Sweden, for example, and Proton is from Switzerland, so if a content company can even figure out which endpoint nodes are hosting/routing the pirate content they then also have to figure out (a) who owns the node and (b) then send them an angrygram which will just immediately be torn up by the VPN provider as they’re not subject to US law.
Finally, an operating principle of these companies is to keep no logs, so even if a US-based VPN company got an angry letter, they’d probably be unable to do anything since they would have no record of the activity.
Related, Pirate Bay used to (might still?) have a section where they mock all of the threatening letters that cite a different jurisdiction. Usually the US DMCA, but also similar laws from other countries.
They never posted any letters that cited Swedish (IIRC) law, because those were valid threats.