We've seen this before, it's a bit like the "no outside food or drink" policy at the movie theater. Apple (and Google, Microsoft, et. al) want to stop you from using your media so you're funneled into buying theirs. If people can easily play local files, it diminishes the value of a streaming/music management tool. Probably explains why Groove Music and Play Music failed, while Apple Music still has a surprisingly devout userbase.

This is one of those things that will hopefully be opened up with more standardization. There's nobody to blame but ourselves for not enforcing this earlier, and hopefully in the future opening an MP3 file won't be harder than it was in 2005.