1. Instead of needing 100 passwords, which increases the chance of users just choosing something and repeating it, you have 1 password.
2. Similarly, instead of needing 2FA on 100 sites they can just have 2FA on their SSO. In fact, the other sites don't even need to support 2FA - you get that "for free" with SSO.
3. SSO providers implement auth really well. They make it smooth, as in "I don't have to reauth when it's obviously me" and safe, as in "that might not be a valid auth, let's get them to 2fa again".
Of course, if you have a password manager then (1) is not a problem. But SSO is a lot simpler for users.