There’s a big risk of false positives. For example, “hot” is just a normal word and should not be blocked (e.g., the stove is hot). Nor should “breaststroke” be blocked for someone who wants to learn swimming.
For HTTPS sites, you have to break the encryption in order to filter by words (i.e., a Man in the Middle attack against HTTPS). We will not break a secure protocol to make the Internet “safer”.