People are building her just fine, this is not a valid reason for her winrate being so low.
Playing he wrong is definitely a reason for lower win rates.
Being only decent early game and great mid game (where you either split push or team fight) means she is more team dependant. What means solo queue will mean a lower win rate for her. Sure, she can 1v1 without problems, but her biggest plays are in teamfights.
If you mean TriForce, GA, Wit's End and BC are all that great for her.
Sure, the items are good, but why whould you need more MR when you can build more HP and your team gets a Bullwark?
The statistics show that there are good builds with her, but still too many people are using the old build (in around 60% of the games with her people want to get a TriForce ASAP).
And flask on Irelia? after the nerf it is not worth it.
And no Doran's blade late game? There should be one in your inventory left. IT is the best early game item for her. If you have the money, Dorans blade + Zehpyr will give you a lot of offensive power and sustain. Or Warmogs.
Dorans blade is now way better for her than the old one. She doesn't build a lot of AD, so the old % LS was bad for her. the new flat thing is way better when you take some AS. And cause she builds not much AD, it will never get worse than the old dorans (like it happens for ADC or AD casters).
The problem is not what you are building but what you are building in certain situations.
You have no ONE OVERALL build that works always anymore (except for the HP builds).