为什么避免在测试类中引入共享状态不好?

我正在读一本关于单元测试的书,下面在书中被表示为错误代码:

public class CustomerTests
{
   private Store _store;

   [Fact]
   public void Test1()
   {
      //act 
      _store = new Store(5);
      ...
   }

   [Fact]
   public void Test2()
   {
      //act 
      _store = new Store(100);
      ...
   }
}

As we know that when xUnit or other testing frameworks, a new instance of test class is created(constructor runs before each test in the class) for each test, so in my example, since I have two test methods, so there are two instances of CustomerTests, since they are different object/instance, so _store instance/property in different instance are therefore also independent, which means one test methods modifies its _store, the other _store in other instance is not affected. So they technically are not "shared state", why we need to avoid that?