Because it’s not the basic Wireguard part itself that’s hard, it’s the synchronized keys, NAT busting, ACLs, and other things on top of it that are. Tailscale has made it incredibly easy to deploy and use confidently and that’s why people are happy.

Your comment has strong "what does Dropbox do that curlftpfs doesn't?" energy.

For exactly the same reason that Dropbox was just "rsync with a shiny UI" and was very popular.

It's less about the UI and more that Tailscale takes care of setting up wireguard connections between every machine that needs them, along with the authentication and access control. All of which ends up with a simple network between your devices that doesn't have to worry about physical location and intervening network infrastructure.