Because those sites generate codes using old algorithms or reuse the same three leaked codes. Vidmore updates its server blacklist weekly. Legitimate codes are unique to your email and purchase.
Videos claiming to offer free codes are scams. They usually ask you to complete a "survey" (stealing personal info) or download a password-protected archive (containing malware). Avoid at all costs. vidmore player registration code
If you work in education or a non-profit, contact Vidmore sales directly. They sometimes offer discounted site licenses. Because those sites generate codes using old algorithms