Its a DaaS. That's literally what it's for. That's like saying, why would you use the cloud? That's silly. Host it in your closet like we all did in the 90's, because that never caused any problems... Or what, use firebase only for chat data? So when you leave the company, some junior has to figure out 10 different places that the data is coming from when his boss starts breathing down his neck? Sounds like a freakin nightmare. There's nothing wrong with trying to keep things simple.

That worked really well for them! They obviously stored data in Firebase that did not belong in there. I have nothing against Firebase, but it's not meant to store all your app's data!