Why are most popular songs, as heard on the radio or a streaming service, approximately three minutes long?

You could argue that it's because the early records were only able to fit about 2-4 minutes on them for "singles" which musical artists and studios would send to radio stations for play.

I find it interesting that in 1958 the Eurovision song contest limited all songs to 3 minutes, so it may just be arbitrary and expected after the technology caught up that hit songs last ~3 minutes. In fact, the vox article mentions that anything significantly longer than 3 minutes usually just gets a "radio edit" and that's because songs too long cut into profits either of the radio station (can't run ads if the song is running), or the record label ( music royalties aren't based on song length, just plays).

https://en.wikipedia.org/wiki/Three-minute_pop_song

https://www.vox.com/2014/8/18/6003271/why-are-songs-3-minutes-long