The reasons change from person to person. But in my case, there are a few reasons (sorry in advance for the wall of text and if some of my comments are a bit heated).
First, programmed obsolescence, such as that as soon as Google’s validation system stops supporting a certain OS, the user won’t be able their games from Play Store in that OS, even if it the game would run perfectly fine.
Then there’s preservation. Some games I bought aren’t available DRM free anywhere else (at least not legally), so my backup is the only legit way I can play the game.
Also, if an update breaks something in the game, rolling back to a previous version, at least from the user side, is a nightmare with paid apps from the Play Store. Also they have auto updates set to on by default, and that makes things even harder to manage (you can disable it, but if you need to reset Play Store’s data for whatever reason, they’ll update every app you have as soon as you connect to the internet again).
And I never rooted a phone, but I imagine I can save some precious RAM and storage size by removing Google’s services, and that’d be specially good with some older phones. But by doing so, I’d lock myself out of Play Store’s apps.
Also, I remember reading that Google can be, in the lack of a better word, “volatile”. From what I read, even the Google account of the dev of Terraria was banned out of the blue. And good luck getting in touch with Google’s fabled support if anything happens. And in the meantime, the user who lost access to his/her account won’t be able to play their games. (in fact, I couldn’t even play some PS1 and PSP games from the PSN in the Vita some weeks ago due to those DNS issues that broke a lot of sites)
Then there are other problems with Play Store’s layout and functionality. Some times it takes hours for Play Store to understand you bought/activated a product across devices. Their library system can be worst than IndieGala’s, and that’s quite a feat. Their search system uses more of an AI than an exact search system, making it hard to find certain apps. Their home page is full of games with microtransactions. Their wishlist system feels worse than Itchio’s (again, quite a feat).
And lastly, while not as big of a problem on Android, running on other platforms games that the store has full control over usage can be rather complicated, to the point even Valve/Steam started saying developers shouldn’t use DRM for better compatibility across OS’s (source)