Partly because for a long time Android allowed apps to do their own billing for subscriptions and other IAP, as long as the content you purchased could be accessed on other platforms (with a big carveout for games, which always had to use Play Store billing for IAP). That's why Spotify railed against Apple for years but didn't mention Google.
They changed their rules a couple years ago to make it pretty much consistent with Apple's policies, but I think some of the bigger players got exceptions.
They changed their rules a couple years ago to make it pretty much consistent with Apple's policies, but I think some of the bigger players got exceptions.