because if you get your account compromised on one site/forum because of lax security on their end, you don't want all your accounts compromised.
How will your accounts be compromised if they are all different? A key is no good if you don't know which lock it leads to.
Because it might be tied back to your email account or some other common denominator. Using different usernames is good for dox attacks like this but doesn't necessarily help you if your email is compromised. So you need different passwords (AND MFA!!!!)