Because it is. Just recently some VPN provider I already forgot the name of cancelled all lifetime subscriptions. There were multiple posts on Lemmy about it.
Ten years and counting for me and VPN Unlimited so far. Unsustainable? Capitalism is un-sustainable. Cheaper than paying monthly or yearly? Absolutely.
Personally, I use the VPN to do my searches, grab magnet links, and get the torrent started, then I disconnect the VPN and switch to Cell data, as that ISP has never sent me a nasty-gram about pirating.
Reasonably sure Nord is still turning a profit on my sub.