Why can't Spotify allow users to randomly shuffle through songs?

I don't know what really goes on, but it's pretty clear the shuffle function does not randomize. It has preferences and will repeat some songs multiple times and ignore others. First time I realized was in a month I was only listening to a single playlist, with around 100 songs, and then I was missing a song, which I haven't heard for long time, only to notice it was indeed in the playlist but being ignored? What's even the point of creating playlist if Spotify won't lemme enjoy it fully? I'm seriously considering going back to mp3 and external harddrives and unplug my premium spotify account. Is there any alternative in the market with a similarly large collection? Is this a bug? What's going on here? For at least 18 months I have noticed it's a consistent behavior across all my devices (up-to-date software).