Este artigo detalha muito bem e tem a maioria das informações que você está procurando. A citação-chave da minha leitura é
The simple answer is that using regional airlines saves major airlines
money compared to them operating a flight themselves. Where do the
cost savings come from? For one, employees are paid significantly
less.
Também existem sérias razões históricas e sindicais para a instalação, o compartilhamento de códigos das companhias aéreas regionais com empresas principais é, de certa forma, uma maneira de contornar as negociações sindicais
The first reason is that it would likely eliminate the cost advantage.
Most major carriers have negotiated with their unions that flights
with fewer than “X” number of seats will be operated by regional
airlines. You can bet that if the regional airlines became part of the
major airline (even if a separate division), the major carriers’
unions would be fighting for significantly better pay for those crews.
...
Historically another reason the major carriers have contracted out to
regional carriers is because it allows them to share some risk. Keep
in mind that historically airlines are ridiculously unprofitable
(though that has changed recently). So leased planes were almost a
liability rather than an asset, given that they were losing money with
just about each flight.
Aqui está outro artigo interessante sobre o assunto e boa informação aqui.