I don't know how to make this simpler.
Yasuo has mobility. Mobility comes with the potential of outplays. Outplays are more facilitated the more you are able to do them. You are able to do them if you have the health and medium damage to do so.
Darius sits as a juggernaut who dunks melees that want to punch his armored chest for some reason. Yasuo, can in fact punch his chest when he wants and get out so that Darius passive, which when max stacks becomes a full item meant to counter tanks and melees, cannot be abused, and can make trades as the Yasuo wishes instead of as the Darius wishes.
If you believe your items always make up for your champions abilities, then you are going to have a bad time. Items are there to facilitate your abilities and playstyle. Not me big number, me smash. Although granted, some champions can do that if you let them.
Let's take another one. I can be Jhin. I can have 2 items up on Camille or Vi. She will still retain a possibility to kill me. Does that mean I am not useful? No, I can murder their team under the right conditions, and many other champions in the game with ease. But a champion who abuses their kit can abuse another, especially if they build to do it.