It seems to me the parent commenter is saying the opposite: looking exactly like each other _is_ the point. It's a form of social signaling, to indicate that a project "belongs" to the in group of high-flying successful AI hype projects.
Note I'm not arguing that this is a good strategy. But given that so many people follow it I imagine it's not as bad as it appears on the surface.
It's a bit of in-group signaling but I think, importantly, also date signaling. A 2026 hype website looks different from a 2020 hype website looks different from a 2010 hype website. Having a generic 2026 hype website look tells visitors that you're either new or update your website's design to follow current trends.
They do the same with cars, where it's even more important and even more explicit. The design language has to change every couple years so that you can tell when somebody is driving a car older than 5 or so years. For example, currently we're doing blobs but with a few sharp features and muted colors. Before that it was more colorful and more metallic paint. Before that, in the 00s, it was pure blobs. Before that it was all sharp edges etc. Now sharp edges are beginning to make a comeback.
That's why I don't think we'll ever have the "one true design language". Fads and trends will continue, repeating themselves to a degree but also changing in new ways.