Because it's awful. A normal flow for me is credentials stored in browser/password manager. Login is more or less seamless, and typically takes under a second.

Because it's awful. A normal flow for me is credentials stored in browser/password manager. Login is more or less seamless, and typically takes under a second. OTP login is multiple steps, involves me doing a copy/paste (or remembering the code), and requires a mandatory delay while I wait on the email. If I wanted to login incognito, or in a different browser, I may have to copy/paste the URL etc. A better question is why anyone uses them at all given how bad they are. I've stopped using at least one site because it exclusively uses OTP links.