What Ghibli films don’t have plots? Ponyo, Totoro, Arrietty, Poppy Hill and Marnie have plots. Whisper of the Heart? I don’t remember the plot of that one.
Tales From Earthsea’s problem was the predictability. The course of the story was formulaic. Ooh there’s this evil wizard who must be defeated, big deal. Ghibli characters are usually more complex. Villains should have something noble or beautiful about them to get some of the viewer’s sympathy. When the protagonist was imprisoned and then the friend came and saved him, it was just too predictable. The rescue could have been done artfully but it was not.
The villain of Castle in the Sky was similarly boring, but the colorful supporting cast made up for it.