Accessing Table and Column Comments in Oracle for Tables in Sql Server via Databaselink

I use an Oracle database, where I have a database-Link to a Microsoft SQL Server database. I need to access the comments for tables in the Microsoft SQL Server database from my Oracle database.

Using the script below I get the values for owner, table_name and column_name, but my comments field is empty (null), although there should be comments.

Why can't I query the comments?

select owner, table_name, table_type, comments
from all_tab_comments@DB_LINK_SQL_SERVER;

select owner, table_name, column_name, comments
from all_col_comments@DB_LINK_SQL_SERVER;