The danger of using the same password everywhere is from leaks caused by poor security in one of those sites.

Passwords getting leaked are almost always unrelated to how strong the passwords are and has more to do with how those password are stored, and what protection measures they have against unauthorized people accessing them.

No one is ever going to "crack" your single password for your password manager as long as it is a strong password, though you might write it down in your wallet and lose it in a busy station, just like some administrator of a website might forget to close outside access to their mysql database containing unencrypted plaintext passwords.