I don't understand this whole "don't blame others" for life circumstances

Isn't it grandiose to only blame yourself? But then they say its narcissistic to blame others for ones life outcomes or choices at the same time. Like what is it?

You could argue something like "its others fault but you were naive to have been tricked by them" or lacked the power or something, but I don't see why I can't blame others when I'm just 1 person.

In my opinion, it 100% is for the most part others fault for my life outcomes, I'm only a small part of what influences the world. At most I'm at fault for being naive that the world is a good place, likely from narcasistic delusion or what not.