Let's start with a story
Think about the following: You buy a coffee for $2.99. After tasting you notice, that it misses sugar. You tell the barkeeper about it and get free sugar. Then you notice that there are also delicious cookies. You ask him if you could get one, because you already paid for your coffee. Well you don't? Why? It's because you just paid for the initial product - your coffee. You already got free sugar, but you can't get everything for free over time, just because you paid once. The same shall apply in cases where you buy software.
You buy a product, but want free maintenance, new features and support, as well as use offered services. The only way for it to work is a subscription to an app.
Because it's the only way to make a great product
With a single In-App-Purchase you can get the following
This is great, but software changes. The development isn't done automatically by robots. Real humans have to spend hours to adapt the software to the newest operating systems and demands of its' users.
Only subscriptions allow to offer the following things
If the products really enrich your life, you could think about a cheap subscription - and think about spending less money on things you don't need or even violate your health. But at the end it's your choice. You can try all products for free.