IMO, Meyer was never a realistic option. I thought Petersen was worth reaching out to, and I think Trev probably did. However, those coaches need to have compelling reasons to come out of retirement. Part of that is walking into a situation that wouldn't require a lot of work, and ease of recruiting. I don't think Meyer or Petersen would want to take part of a complete rebuild, like which is necessary at NU.
Before someone says "Well, Meyer and Petersen could attract 30 transfers and flip the talent immediately," that potentially could be true for Meyer, but I don't think Petersen is a big enough name for kids looking to play college football. He last coached in 2019, when most players coming into college in the next couple years weren't even in high school. Yeah, Petersen was at a big school in Washington, but it's still isolated in the Northwest, and kids outside of the West Coast probably don't know much about him. He doesn't have the national brand name that you may think he does.
Simply put, Trev may have reached out to both of those guys, and then those guys call around to coaches they know and ask them "How big of a turnaround job is Nebraska?" and those coaches respond "You don't want to go there. That place is a mess." so Urban and Petersen are like "Nah, I'm good in retirement." I know throwing money at guys can attract coaches, but there also has to be other compelling reasons to get coaches to come to your school.