Why mentioning NOT NULL while applying PRIMARY KEY constraint?

Question

According to my experience with databases you don't need to do so!

Answer 1

Just an extra precaution to make sure it always has a value. Can really mess things up otherwise.

Answer 2

I know what's the meaning of PK, my input was about that PK = NOT NULL + UNIQUE + INDEX
why they need to mention NOT NULL twice at the table creation?