Another reason for this is that this kind of asset is way easier to implement than full blow realistic 3D models and environment.
In the end it's all about that, sacrifying game immersion so to be appealing for casual gamers who are not interested in game immersion and could even be put off by that because it contributes to let them think game is harder to play because it's not 'just a game'.
I am just saying that for this kind of games, where the hype/fun/immersion is all about how much fine-grained procedurally generated spatial/life details you can get, to make it all look cartonish style it's a bit simplistic and not up to the capability/deepness/complexity of the ecosystem engine they are proud to market.
To me it's just a bit of riding the trend because indie devs know very well that nowadays cute looking cartoonish indie games are well praised and boosted by mobile app stores whose ratings are driven by casual gamers looking before everything else for cute bleeps and rainbowish color splashes!
Anyway this is just my little opinion and I am sure it is incomplete and does not encompass all the aspects there are to it. Just see it as a starter for a more in-depth conversation, if you wish.
Provided you will take a 360 degrees approach to it :)