如果我正确理解了您的问题，原因是Button源自ContentControl，ContentControl只能包含1个项目.

不过，它不一定必须是网格，它可以很容易地是StackPanel或可以容纳多个项目的任何控件.

If I have understood your question correctly, the reason is that Button descendss from ContentControl and ContentControl can only contain 1 item.

It doesn''t have to be a grid though, it could as easily be a StackPanel or any control that can hold more than one item.

好吧，我建议您使用一个边框而不是一个矩形.矩形只是一个形状，而边框是一个容器，因为它继承了Controls.Decorator类.
在此边框中，可以将ContentPresenter放置在其中，而无需网格来固定它.您不能对Rectangle进行相同的操作，因为它不能容纳元素.

因此，您可以像这样创建边框:

Well i would suggest you to use a Border instead of a Rectangle. Rectangle is just a shape but a Border is a container as it inherits the Controls.Decorator class.
In this a Border can place the ContentPresenter inside it without the need for a Grid to hold it. You cannot do the same with a Rectangle as it cannot hold elements.

So you can create a Border like this :

<ControlTemplate TargetType="{x:Type Button}">
<border name="ButtonBorder" cornerradius="4" borderthickness="1">
   <contentpresenter content="{TemplateBinding Content}">
      VerticalAlignment="Center"
      HorizontalAlignment="Center"/>
</contentpresenter></border>

您可以在我的博客中查看:圆形按钮 [ ^ ].

我想补充的一件事是，矩形"将仅为您的Button提供一个形状，而ContentPresenter将显示Button的实际内容.
例如:

You can check it out here in my blog : Round-Edged Button[^].

One more thing i would like to add is that, the Rectangle will only give a shape to your Button but the ContentPresenter will display the actual content of the Button.
For Ex:

<Button Name="myButton" HorizontalAlignment="Center">
   <Button.Content>
       <Image Source="Winter.jpg"/>
   </Button.Content>
</Button>

如果要在按钮中显示此图像，则在模板中，必须设置

If you want to display this Image in your Button, then in Template, you have to set the

<contentpresenter content="{TemplateBinding Content}" />

希望我能解决您的疑问.

I hope i was able to clear your doubt.