Why did rent used to be so cheap?

I was watching The Apartment (1960) and the main character, C.C. Baxter said his rent for a 1 bedroom apartment in the Upper West Side of Manhattan was $85 per month and he even implied that it was somewhat expensive in the context.

He also mentions in the movie that he makes $94.70 per week as a clerk, meaning he can easily afford his rent, being that it's only like 20% of his monthly salary.

Even accounting for inflation, his apartment is just $900 in 2024 dollars. Rents for that same apartment would be at least $4000-5000 per month now.

What I don't get is that the UWS has always been an affluent part of the city, so there's no way it was gentrification or something. The population of Manhattan was also higher in 1960 than today. Why was housing so much cheaper in 1960 even when wages and inflation are taken into account?

Thank you for your answers