Why are the elements not centered?

Question

XAML

I am writing a UWP app.
There is the following markup:

<Page
    x:Class="Fiphoto.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:Fiphoto"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="1*"/>
            <ColumnDefinition Width="3*"/>
            <ColumnDefinition Width="1*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="60"/>
            <RowDefinition Height="60"/>
            <RowDefinition Height="60"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Button x:Name="buttonSendPhotos" Grid.Row="1" Grid.Column="1" Content="Send photos" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
        <Button x:Name="buttonGetPhotos" Grid.Row="3" Grid.Column="1" Content="Get photos" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>

    </Grid>
</Page>

I launch the application on my smartphone, rotate the screen and see the following:

How to fix this?