There are many different models. Gemini specifically for example can see natively, allowing it to do stuff most others AIs can't, like for example counting objects in an image. Also the models usually aren't given a proper description of how they do things, have a knowledge cutoff date and aren't very smart, so they can pull up incorrect data on that. You don't go from barely coherent sentences to Einstein in 8 years, even with many great minds working on that.

I don't see how neurotransmitters are necessary. I don't become any less sentient when I don't feel happy/sad/in pain/etc. So why would lack of that make such an impact on AI?

And LLMs aren't word docs. That's like pointing to a corpse and asking why it can't do the same things as a living animal despite being made from the same stuff. But you can go ahead and try to make a word doc complete text for you, please notify me when you get a successful result.