Currently there is a limitation in Google BigQuery connector: "The ability for Tableau Desktop to query nested fields in a Google BigQuery connection with Standard SQL is not a current functionality of our product"
Accordingly to Google users should migrate to Standard SQL ( Migrating to Standard SQL | BigQuery | Google Cloud Platform )
"Migration from legacy SQL to standard SQL is recommended but not required. For example, suppose that you execute many queries that use legacy SQL, but you want to take advantage of a standard SQL feature for a new query"
Accordingly to Tableau best practices on performance user should use Standard SQL: Google BigQuery & Tableau: Best Practices
Google BigQuery upgraded its APIs to use standard SQL in addition to BigQuery SQL (now called legacy SQL), and Tableau upgraded the Google BigQuery connector to support this change to standard SQL. With standard SQL comes benefits for BigQuery users including Level of Detail Expressions, faster metadata validation, and the ability to select a billing project with your connection. This guide is written assuming standard SQL
Both parties recommend the usage of Standard SQL and we can't because there is a limitation on Tableau side.