I know I am sentient, so it stands to reason other humans are sentient. Additionally, other humans act in a way I would expect from someone sentient. Neither of these apply to any existing AI.
That doesn't stand to reason at all, it's just popularly assumed.
I can hear my thoughts, but I can't hear yours. I can see through my eyes, but I can't see through yours, and so on. There are many differences between "I" and "everyone else".
Just because something is true about you doesn't mean it stands to reason that it's true about everyone else.
> Additionally, other humans act in a way I would expect from someone sentient.
This is totally circular reasoning. Humans only act the way you expect sentient people to act because your training data for "sentient people" is "humans". The only person you know is sentient is yourself. Personally I find that most people quite frequently act very differently to how I would act. Should I conclude that they're not sentient?
Further, the fact that machines don't act like humans is no evidence at all that they're not sentient, it's only evidence that they're not human. They are very different to humans, so of course they don't act the same. They don't have bodies like us, or biological needs like us. That says nothing about whether or not they're sentient.
1. Other humans are sentient, therefore they act like what I would expect sentient beings to act like
2. Other humans (or a subset of them) are not sentient, they are just insanely complex biomachines that mimic acting like what I would expect sentient beings to act like
In 1. sentience can be explained as a property common to at least humans, and maybe further as an inherent property of complex machines.
In 2. sentience seems to be a property of a subset of humans. Why? What makes sentient beings sentient? Answering this question requires invention of a soul.
Theory 1. feels simpler to me.
Yes, this is the meat and potatoes of the question. Personally I think that the safest approach in that case is to treat them as if they were sentient, but there are lots of people with various prejudices about what the prerequisites for sentience are that would fall on the other side of the question.