Because we cannot accept a certain piece of software as finished.
It doesn't mean there aren't any updates. However, in the iOS store I can update some apps every week.
On the other hand, I very much like the F-Droid store. There are so many useful and user-friendly apps. They work and stay like this for a long time.
I suspect the underlying topic here is money. Open source apps don't get much funding. So the developers need to focus on the essentials and get them right. On the other hand, subscription-based apps have a steady inflow of money. For some reason, they need to constantly work on these and add new features with marginal utility. I regret having updated some iOS apps. They were working perfectly fine in 2018 but have added bloat and bugs since then.