Why do only constant constructors in Dart, not non-constant constructor, can't assign different type of value? – Flutter

I’m getting error "A value of type ‘Null’ can’t be assigned to a parameter of type ‘String’ in a const constructor." from const Text().

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title, required this.subTitle});
  final String title;
  final String subTitle;
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
   return Scaffold(
      appBar: AppBar(
        title: Text(
          widget.title,
          textAlign: TextAlign.center,
        ),
      ),
      body: Column(
        children: <Widget>[
          const Text(
            widget.subTitle,
          ),
      ),
    )
  }
} 

I already know I have to remove const to resolve the error. But Could anyone explain why does only const constructor have this error?