I can't tell you if there actually were or were not so many pianos in the streets, but I can tell you there were a lot of pianos that could end in the streets.
The piano was important for the middle classes as something that was part of the kind of life they wanted ("we are doing well, we can have nice things") and because in order to have music at home you pretty much had to play it yourself, or pay for some musicians to play. There were pianos in churches, schools, pubs, hotels, theatres, etc. Player pianos (and machines to play the piano) were sold, lots of them. They became popular before the radio took over.
So, a lot of pianos were sold because a lot of people wanted them, and pianos are pretty durable (grandparents can give their pianos to their grandchildren if they take care of them).
If you were emptying houses, there were very good chances of finding quite a few pianos in the neighbourhood.
Today a piano is not a rare thing to see in the US-Canada or Europe (and it's kind of becoming acceptably popular again in Asia), but piano sales have dropped significantly since the 1930s. The piano was way more common back then. What happened? Well, the radio changed musical life, and there were also the economic implications of WW I, the Great Depression, WW II, then the TV, and then more.
hi! I suspect this question may do better in a sub specializing in dramatic tropes (assuming there is one), but anyway, you may be interested in responses to this similar question