but at the time, it's all there was so it was important.
Bingo. Music back then was much more important in selling a movie since there was no internet and no social media. It also helps explain why movies with very memorable soundtracks are much less common today than they were in the past.
I seem to remember a documentary or some such, stating that it is actually a trend started more or less with The Graduate
from 1967 with Dustin Hoffman, and the soundtrack using some Simon & Garfunkel songs. Speaking of non-musical themed movies using air-played songs, so excluding all the Elvis, "Singin' in the rain" etc. movies made to promote the musician/band....