The Specific is Covered in the More General
Leviticus 18:17a states (NKJV):
You shall not uncover the nakedness of a woman and her daughter
If a man has a daughter, he certainly (at least in that time before in vitro fertilization, etc.) has experienced sexual relations with the "woman" who is the mother of both "her daughter" and his daughter. Now 18:17a would generally include stepdaughters as well, but that general statement also includes the specific case you are inquiring about of one's own daughter. So there was no need to specify "one's own daughter," when this more general statement already included that.