Lots of mainline pilots never flew at the regionals. Military at one time was a bigger portion of the newhire classes. Of course in the 80’s and 90’s the major carriers might hire 300 in a great year so there was less of a need for the majors to hire from a regional supply chain. Keep in mind there was not a lot of hiring in the 70’s due to oil, regulation and economy. The 80’s had the failure of Braniff and Eastern with Pan Am, Midway I (please a lot of other small carriers) in the early 90’s. Jobs were scarce and highly competitive. Pilots faced rigorous interviews versus the fog and mirror get hired these days.