This is already somewhat out of date given how fast things are moving in this space, for example with the amazing work being done by antirez on the DwarfStar 4 to run agents on a quasi-frontier class model on consumer hardware.
He a self important egotistic who's finally admitted he's been wrong the whole time and "discovered" what's been under his nose for over 20 years after all the people he kept shitting on did all the hard work to make Linux a fantastic option for dev work.
Because your app will work on all platforms and the rest of the billions of app users outside of this little niche-of-a-niche-of-a-niche forum just want to do whatever they need to get done on a app in their normal lives and don't even know what you are talking about when you say the app "sucks" because it doesn't use exactly the right rounded corners on a button or the listview scroll physics are exactly same as a "native" app.
Because you'll still want to support both OS's sooner rather than later and then you'll be re-inventing the wheel by rewriting all your code a second time on the other platform. Plus using Flutter from the start means you have the option of seeing a much wider world than just Android or iOS: you can build the same app for Linux, MacOS, Windows and the Web.