Because if royalties are function of licensing volume, it makes no difference who you choose. If more people ditched Spotify for Apple Music, they’d start getting more favorable rates with record labels.

If you really wanted to help artists, it would make the most sense to use whichever service is cheapest and then donate the difference to artists. But, personally I’d just go with whoever provides the best user experience, which is none of them, so I basically haven’t listened to music aside from occasional YouTube chill beats for years.