The thing is that your argument is just not compatible with real life. Humans are humans, they are absolutely not going to create unique and strong passwords for every service.
What happens in actuality is that they create one or two okay passwords, then reuse them, which is 100,000x worse than using a password manager.
The promise of a password manager is you get infinite perfect passwords. Perfectly long, perfectly unguessable. And then you can redirect your brain power to making one really, really good password you can remember. The benefits are plentiful. You don't have to worry about breaches. You're immune to dictionary attacks and brute forcing. And, it's much more convenient.
