Get an extension cord?

…sure. Yes. If you own a song, you can listen however many times you want, simultaneously or not.

But streaming services are simply a different value proposition. Listening to an mp3 means either buying all the music you listen to or pirating; it also means having the music stored on your listening device in advance, or streaming from a personal media server. I listen to a lot of music that I haven’t heard before and don’t know if I’ll actually like; I also listen on my phone a fair amount and have a limited amount of storage space for music. For that use-case, streaming is preferable (to me).

Maybe, but how is that different from needing to pay for two separate copies of anything else if two people are using them at the same time in different places?

I’m not a fan of how little the major streaming services (except Tidal) pay artists, but they do all offer bundle packages. Spotify’s pricing is $12 for an individual, $17 for two people, and $20 for a family of up to 6. So it’s only $5 more than the base cost if two people stream simultaneously.