It isn't necessary for a unique password for every account. There's no need for me to have a unique password on every gaming forum - for example.
Different passwords for important things, yes, of course. But for trivial ones (like forums) - no.
The reason is that unless people use password managers then they tend to get overloaded with the passwords to remember. Because of this they then select easy to remember passwords. I don't mean they choose password123, although statistically still chosen a lot, I mean that they start using repeatable and therefore flawed logic in password choice. Such as adding numbers - corresponding to their date of birth. If they have to change a password regularly, they generally add 1 to the end of it each reset time - i.e. password1, password2, password3... etc
So having 20 gaming forum accounts with different passwords, which are then identifiable via my email address amongst leaked lists, will potentially reveal my password strategy. Nearly all of us have one unless you use password managers exclusively. Most people will have patterns in how they constructed passwords. Birth date years, old addresses, telephone numbers, post codes, car plates, pet names, siblings, favourite personalities, or the inclusion of colours, or a theme of some description. It's how our brains work, makes us predictable, and therefore easy to defeat.