I don't have anything to back this up, but my guess is that the vast majority of compromised user accounts comes from credential stuffing/password re-use. It's really surprising to me when I hear that huge companies don't do this check.[2] It's simple, easy, takes about a day to set up.
If you're a young CTO or early-stage engineer working on a web app and have never been targeted with a credential stuffing attack, let me tell you: It's coming! It's just a matter of time before it's 1AM and your phone blows up; your site is getting hammered; you think it's DDOS, but then realize most of the hits are on your login page, then realize that and then realize with a horrible feeling that some % of those hits are getting through the login page. You'll be up all night dealing with it, and then you have to make breach notifications, and that really sucks.
Troy Hunt's free database will save you that heartache (probably). Just do it.
1. https://cheatsheetseries.owasp.org/cheatsheets/Credential_St...
2. Like 23andMe. https://news.ycombinator.com/item?id=37794379