Unless you tape-delayed, you
had to hit the network on time or it sounded like hell.
The stations that did their own news that were obsessive about it tended to appeal to an audience that was used to "news at the top of the hour", and in many cases, competed with network-affiliated (or network-owned) stations.
KMPC wanted to have their newscast start at the same time as KABC and KNX. But even then, if Whittinghill or Gary Owens ran late, you'd hear the time tone in the middle of a commercial and the news would start anywhere from a few seconds to a full minute after.
Listen back to airchecks and you'll hear plenty of times where Robert W. Morgan was running late, hit the :36 break at :38 (timechecking as he went in), played a record out of it and the news began at :42...but the newscaster still said "It's 20 minutes before 7, this is (name), KHJ 20/20 News."
I mean, is it? Where?
As
@Kelly A says, news is a scheduled event on the program log. Barring an extreme circumstance, there's very little "It's 6:03 and time for the 6 o'clock news". And nobody's attitude is "when we get around to it or when we feel like it".
Part of it is actually tech---it's HARDER now, with modern software, to adjust stopsets and song lengths on the fly.
As a jock, I'd start backtiming in my brain at :50 and give myself plenty of room to stick the landing at the top (before we started recording the :54 and playing back). Making the "executive decision" to ditch a promo, skip a jingle or move a commercial into the next hour to hit the mark cleanly was no big deal.