Complexity for one. A cramped foot has an influence on the brain, as does apparently the gut bacteria. Focusing on the brain is a starting point and we don’t even understand that that well.

If someone perfectly simulated your entire brain, would that digital brain be sentient?

I don’t know. It could be. For now I don’t think so. Are you comparing that to an LLM? That would be like comparing the paths of snail slime to a comic. One could compare story lines and art styles to something that just isn’t there. And never will be.

What is sentience?

Sentience is the ability to experience feelings and sensations (wiki). A word not based on a clear understanding, but rather an attempt to categorize. Nonetheless, an LLM doesn’t experience anything. It uses pattern recognition and human provided categorization to try and create different stuff. All in the confines of the recognitions.

I think it’s strange to say that AI will never be sentient.

It’s why it’s important to distinguish between “AI” and “LLM”. AI, as an AGI, is something we might be able to build one day. LLMs might be a step on the way to this. But not the way they are now.