If the OUTGOING INTERFACE is already mentioned in the routing table, it is natural that the next hop will necessarily be the router attached to the outgoing interface. Then why is there a separate coloumn for the NEXT HOP?
The frame on the outgoing interface needs to be addressed to the next hop. For example, if the two routers are connected via ethernet, there may be multiple other devices on the ethernet between them, and the frame needs to be addressed with the MAC address of the next router.
For other types of data-link protocol, there may be other addressing involved. Frame relay uses DLCI, and you need to get the DLCI of the next router, or with ATM, it is VPI/VCI, etc.
If you do not know the IP address of the next router, then you have no way to find the frame addressing to be able to send the frame that encapsulates the packet to the next router.
Additionally, IP routing protocols conceptualize the next-hop as a layer-3 address (for the same reason Ron explains in his answer.)
There is such thing as unnumbered interfaces and some routing protocols have special rules about what to do in these circumstances.
There is an RFC 5309 titled Point-to-Point Operation over LAN in Link State Routing Protocols that may be of interest to you.