That was the height of the “Tin Pan Alley” era where there were people who WROTE songs for a living. This meant they did it well as those who did it poorly were tossed out fast enough.
Songwriters actually worked on things like getting songs to rhyme, making sure words were singable.
Look at songs like “Puttin’ On The Ritz” with things like
That’s where each and every lulu-belle goes
Every Thursday evening with her swell beaus
Or from “That’s Entertainment”
OK that is cool. Maybe not the best line, but it shows a lot of effort to say it.
In the last 20 years other than Diane Warren, who’s been re-writing the same song every other year, there are no songwriters.
Back then the singers were just a tool. You look at songs like “The Tennessee Waltz” that song was in the first 4 places of the top charts by four different artists (I just happen to be reading a 40s music chart book yesterday)
So many artist back then gave their slant on a song.
So you had well written songs because the writers only had to answer to themselves. They didn’t have to sing and make videos and such.
Plus talented people produced more, because they weren’t paid millionaire wages.
I recall an interview with the Phyllis McGuire of the popular sister group, The McGuire Sisters (“Sugartime” and “Sincerely”) and she said they were very fortunate they DIDN’T make it right away as they came in right when singers started making the huge salaries, and had they made it right when they started, they would’ve made a pittance, compared to what they later did.
When people make more, they have less incentive to produce.
So it was a combo of good professional writers, with a lot of singers giving the unique slants to each record. This made for good music. The songs were also done quickly so this meant there was also a lot of bad songs as well.
Look at people like Britney Spears who has had five or six top ten hits and makes a reported one million dollars per month off of it. That’s a lot of money for someone who really hasn’t produced much music.