It really isn't all that odd that Nebraska fans don't consider many teams to be our rival. We're a blue blood (or borderline blue blood) team. We were definitely still considered a blue blood when all of us over 15 were growing up. There aren't many blue bloods, so there aren't many other fan bases who are that picky about rivals. We're a bit snooty about it but it makes sense. We want our rival(s) to be other blue blood teams, and the only team that qualifies for us is (was) Oklahoma. All blue blood teams other than Nebraska, if we still count, seem to have other blue bloods (or close to it) as rivals.
Michigan - Ohio State, Michigan State, Notre Dame
Alabama - Auburn, Tennessee, LSU
Ohio State - Michigan
Oklahoma - Texas, Oklahoma State
Notre Dame - USC, Navy, Stanford
Nebraska - ?
USC - Notre Dame, UCLA
Texas - Oklahoma, Texas A&M