A great analogy would be a website that asks users to provide their usernames and passwords for sites to see if it’s a strong password or if it’s been compromised. “Sorry, the credentials stouset / hunter2 were found in our database for Hacker News.”
Sure maybe you’re a saint and don’t store or misuse this data. But such a site would in the best case be training users to do a very wrong and dangerous thing. In the worst case you get breached by attackers who do use the collected data to do evil.