Does SystemIndex in Window Search Service query only gets C:/Users/{currentUser}/ directory?

Question

I looking for an efficient way to list all files in the drives. The easy way would be to list files in C#:

static void Main(string[] args)
        {
            SearchFiles("C:\\", ".txt");
        }
public void SearchFiles(string rootDir, string fileExtension)
    {
        SearchFilesRecursive(rootDir, fileExtension);
    }

private void SearchFilesRecursive(string directory, string fileExtension)
    {
        try
        {
            foreach (string file in Directory.GetFiles(directory))
            {
                if (Path.GetExtension(file).Equals(fileExtension, StringComparison.OrdinalIgnoreCase))
                {
                    Console.WriteLine(file);
                }
            }

            foreach (string subDir in Directory.GetDirectories(directory))
            {
                SearchFilesRecursive(subDir, fileExtension/*, foundFiles*/);
            }
        }
        catch (UnauthorizedAccessException ex)
        {
            Console.WriteLine(ex.Message);

            // Handle any unauthorized access exceptions here.
        }
        catch (DirectoryNotFoundException)
        {
            // Handle any directory not found exceptions here.
        }
    }

but this is a super slow way to do this. I came across Window Search Service. I wrote the following code which worked faster but I was only able to print files in C:/Users/{currentUser} directory. The code was not able read other folders in C: drive.

static void Main(string[] args)
    {
        var connection = new OleDbConnection(@"Provider=Search.CollatorDSO;Extended Properties=""Application=Windows""");

        // get everything
        var query1 = @"SELECT System.ItemUrl FROM SystemIndex";

        connection.Open();
        int counter = 0;
        var command = new OleDbCommand(query1, connection);

        using (var r = command.ExecuteReader())
        {
            while (r.Read())
            {
                Console.WriteLine(r[0]);
                Console.WriteLine();
                counter ++;
            }
        }

        connection.Close();
        Console.WriteLine($"Counter: {counter}");
        Console.WriteLine("Searh Ended");
        Console.ReadKey();
    }

The wiki quoted

"The index store, called SystemIndex, contains all retrievable Windows IPropertyStore values for indexed items."

Why are other directories not retrievable using the above code? Can I access files in other directory using Window Search Service?