They could do though. All the smarts of Spotify comes from their APIs. There's been a bunch of third party low-resource clients / integrations for spotify; despotify, mopidy, recently this thing:
https://github.com/hrkfdn/ncspotUnfortunately iirc the neat dynamically generated playlist stuff isn't supported because it's not exposed properly through their APIs. Certainly when I tried to use Spotify's own libspotify its functionality was crippled to static playlist management, searching for stuff, and playback. despotify was in a similar state at the time.
Also - Spotify sucks HARD at doing all the stuff it does, and I blame this entirely on the UI implementation. The service, I love. The apps, I have become to loathe. They used to be nice and quick and responsive. Now? It can't even get shuffle right!! My mid tier smartphone takes 10 minutes to load search results or my own playlists sometimes; something i've encountered across multiple devices. The desktop app crawls on my 24gb 8-core workstation.
Conversely, I can have a 200gb library of music loaded into mpd or foobar2000 or vlc in all sorts of exotic (and higher bitrate) formats and from weird and wonderful local and remote storage and play it all on shuffle and have all the albumart showing and it organised neatly into a searchable library via the id3 tags and have volume normalized with replaygain and it'll do it in under 20mb and has been doing so for a decade.........