Exporting DataTable to Excel with C# and OleDbConnection - Only column names are written

I would like to write a DataSet with several DataTables into an Excel file. Each DataTable gets its own worksheet. It works so far, but only the column headings (= columns of the DataTable) are written per worksheet. Is there a way to debug why the values are not written?

I am also trying to recognise the data types, can it go wrong?

 public static void ExportExcelTable(string filename, DataSet dataset)
        {
            var excelConnectionString = GetExcelConnectionString(filename);
            using (var connection = new OleDbConnection(string.Format(excelConnectionString, filename)))
            {
                connection.Open();

                foreach (DataTable datatable in dataset.Tables)
                {
                    using (var createCmd = connection.CreateCommand())
                    {
                        createCmd.CommandText = CreateTable(datatable);
                        createCmd.ExecuteNonQuery();

                        using (var dataadapter = new OleDbDataAdapter($"SELECT * FROM [{datatable.TableName}]", connection))
                        {
                            using (var builder = new OleDbCommandBuilder(dataadapter))
                            {
                                builder.RefreshSchema();
                                builder.QuotePrefix = "[";
                                builder.QuoteSuffix = "]";
                                dataadapter.InsertCommand = builder.GetInsertCommand();
                                dataadapter.Update(datatable);
                            }
                        }
                    }
                }
            }
        }

        static string GetExcelConnectionString(string file)
        {
            var props = new Dictionary<string, string>();
            var extension = file.Split('.').Last();

            switch (extension)
            {
                case "xls":
                    props["Provider"] = "Microsoft.Jet.OLEDB.4.0";
                    props["Extended Properties"] = "Excel 8.0";
                    break;
                case "xlsx":
                    props["Provider"] = "Microsoft.ACE.OLEDB.12.0;";
                    props["Extended Properties"] = "Excel 12.0 XML";
                    break;
                default:
                    throw new Exception($"Fehler: {file}");
            }

            props["Data Source"] = file;

            var sb = new StringBuilder();

            foreach (var prop in props)
            {
                sb.Append(prop.Key);
                sb.Append('=');
                sb.Append(prop.Value);
                sb.Append(';');
            }

            return sb.ToString();
        }

        static string CreateTable(DataTable table)
        {
            string sqlsc = "CREATE TABLE " + table.TableName + " (";
            for (int i = 0; i < table.Columns.Count; i++)
            {
                sqlsc += "\n [" + table.Columns[i].ColumnName + "] ";
                string columnType = table.Columns[i].DataType.ToString();
                switch (columnType)
                {
                    case "System.Int32":
                        sqlsc += " int ";
                        break;
                    case "System.Int64":
                        sqlsc += " bigint ";
                        break;
                    case "System.Int16":
                        sqlsc += " smallint";
                        break;
                    case "System.Byte":
                        sqlsc += " tinyint";
                        break;
                    case "System.Decimal":
                        sqlsc += " decimal ";
                        break;
                    case "System.DateTime":
                        sqlsc += " datetime ";
                        break;
                    default:
                        sqlsc += " longtext";
                        break;
                }
                sqlsc += ",";
            }
            return sqlsc.Substring(0, sqlsc.Length - 1) + "\n)";
        }