Not because of any high principle: it was there to fill airtime - much cheaper to produce than other entertainment.
It was more than that, the news divisions felt they were a public service above profit, and much of a network’s public image was in it’s news division and news readers.
The game changer was 60 Minutes, which showed that the news divisions could produce profitable programming.
But I’m not sure it really matters. Back before profits were introduced the news divisions at least tried (mostly) to present the appearance of unbiased professionalism.
Nowadays it’s all about what advances the narrative.