Glad you asked! Most people think, writing apps is a weekend’s job. That is, they are cheap to make and free to distribute. So if an app has 100.000+ installs, it will easily be paid off if only a handful of users drop a dime. Somebody else will (probably) do it.
Both assumptions are wrong.
Text Fiction subscribes to the ideals of Free Software. That is, “Free” as in “freedom”, not “freebie”. There are still costs that must be met:
Software development is extremely time consuming and therefore costly, worse yet, all of it is an upfront cost that has to be paid by the developer in question with no guarantee of ever seeing that money back. In case of Text Fiction, this comes down to about three quarters of a year of full time work before even doing the first release. That is $23,250, based on the average american income (software engineers earn more, but lets not be greedy).
You likely got Text Fiction through Google Play. This adds a one time sign up fee of $25 and only hosts the app itself. Webserver rent (to host the story catalog) is another $10 every month.
Curating the story catalog, answering support questions, doing promotion and general maintenance adds another day or even an entire week of work now and then. Put $50 per month on the tab for this.
Let me answer with a counter question: “The Play store is full of mobile games/apps, lots of people are using, but nobody is willing to pay (money) for. Where did that get us?”