It's completely the opposite of "use one password for everything". When you do that any single compromise of a website you have an account on means all your accounts are likely compromised. With a password manager you have a long random password for every single website, meaning a compromise is siloed to just that site.
Even if your password vault is stored on the cloud you're likely using a very secure passphrase for it that has 0 reuse anywhere else, so even if your password vault is stolen it's impossible to brute force.
For a hacker to comprise your password vault it would likely involve hacking your computer, which if you're keeping your software updated is a very difficult task these days without the target user's active help.
Even if your password vault is stored on the cloud you're likely using a very secure passphrase for it that has 0 reuse anywhere else, so even if your password vault is stolen it's impossible to brute force.
For a hacker to comprise your password vault it would likely involve hacking your computer, which if you're keeping your software updated is a very difficult task these days without the target user's active help.