They don't recruit at UNC or WFU. They get applicants from referrals and they look at Wake much different than UNC. I'd guess that UNC places the same number of kids in top firms as Wake but has 5x the student body.
I can speak for one "top firm" and assure you that you're wrong. My company, which competes directly with one of the firms listed in your post for both work and employees, has an orchestrated recruiting effort at UNC, along with numerous other, comparable public schools. There are certainly more UNC grads at my firm than Wake grads, although that probably isn't the case if you adjust for size of student body.