To see which tables have no primary key, run the following script:
SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES
SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
WHERE CONSTRAINT_TYPE = 'PRIMARY KEY'
You will get the following list:
Now it's obvious that the Archive tables have no primary keys.
I guess it was a design choice, you can argue that a table without a primary key is not a relation and of course has no guarantee of being able to identify every row. But, I can see how someone would think that since the only source of data comes from the non archive table that have PKs, so there is less risk.
IMHO that is wrong, but not uncommon.
Since the project is on GitHub, you can contribute PKs to the schema.
Martin correctly commented that these are in fact the temporal history tables, and therefore can't have PKs or other constraints as they are system managed.