Because it's based on narratives. Bitcoin has the strong narrative and the "network effect". Because bitoin keeps surviving, the narrative reinforces itself. At this point surely most people know, even if they're unwilling to admit or fooling themselves, the utility value is basically dead.

The utility value is now a pure gamble that a person tomorrow will pay me more than I paid for it today, thinking that a person in the future will pay them more than they paid. And that can be a powerful enough narrative to keep it going.

But if bitcoin disappeared today, 99.99999% of the world wouldn't even notice, that is how little of a problem it solves.

Anyway, the point is, imagine the scenario if ethereum overtook bitcoin in value. What will this actually do to the ecosystem? In my view, it would be catastrophic to the value of all coins, because it suddenly destroys the narrative of a "store of value" (the last lingering narrative). Any other coin could just overtake ethereum, and out of all the thousands of coins, which one? At that point, I think the whole thing comes down.