As an addendum that PPP link isn't talking IP. It's a gateway to something that talks IP so it genuinely has no option other than to shovel traffic over to the IP-aware node. If it has nothing preferable in its own IP routing table the only option is to fall back to the PPP or, actually, whatever, encapsulation link. Won't be running IP to anything.
A reminder - in many cases the actual 'real' publicly routable IP domain doesn't start until the other end of the tunnel. The device on the othe end of the tunnel advertises the IP connectivity then forwards it across a tunnel to your kit. Your kit does not actually have exposure to IP other than locally as far as talking to your end of the tunnel goes.
Public IPs run over a tunnel are certainly routable however there's encapsulation there, so an intermediate device needs a slightly odd routing table to handle than encapsulation. In the case of the mentioned line in the table that was the line bridging IP and PPP. It cannot provide an IPv4 next hop as it's not an IPv4 route, it's a logical interface route with the encapsulation and decapsulation of traffic pushed through that interface all it can rely on.
Realised I wasn't perhaps as clear as I could be earlier.