Why in flutter, setting property mainAxisAlignment to center for column also center it horizontally?

I’m new to flutter, While practising the row and column widget I encountered this problem. It’s said that setting mainAxisAlignment to center in a flutter column will center the children vertically, Why when I added a row as a children, the column also centers horizontally while the row children does not?

Does anyone know why?

Here is the code

  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
      appBar: AppBar(
        title: Text("Exercise row & column"),
      ),
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[
          Text("Text 1"),
          Text("Text 2"),
          Text("Text 3"),
          Row(children: <Widget>[
            Text("Text 4"),
            Text("Text 5"),
            Text("Text 6"),
          ])
        ],
      ),
    ));
  }

Here is the result