Sentient is a loose term depending upon who and how it is being used. Calling an animal sentient in some circles is a misnomer in general. But, thats a larger topic then what I think your talking about.

AI and the modeling of intellect has been something deemed as just around the corner since the founding of computers. And I don't mean home PC's. When Eliza was built, the designers thought that they could model a human in less then 3 years. Its quite a few years later and we still haven't reached even part of what a human can do.

This isn't to say that it isn't possible, in fact I think that it can be done. Something to ponder as your wondering why it hasn't been reached is why humans "Can only use 10% of their brain". Of course, there are savants that use more, but then they (typically) suffer from social depravity.

Recent (in the past 20 years) AI leaps and bounds have been made while trying not to model a human but instead a "lower life form" such as a cockroach.

Cockroach's are easier to model, they have a set sub-system that has been well described by many specialists (outside of the AI world). It also has an amazing ability to morph to fit the world as it is around it. 1,000,000 year old re-instated cockroaches don't run from light, yet modern ones do. This is an apparent change in their hard-wiring over the years. Looking more at this, we can see that their phobia of light comes into existance about the same time that man made light part of every day life (I think this was about 3,000 years ago but I may be mis-stating).

In short, modeling a human isn't as easy as you would think. But, like any development starting small and building up may prove to be the solution. Even when we do start modeling humans, it won't be full grown adults at first, it will be children or infants and their reactions. LOUD SOUND = Bad, soft soothing sound = good.

Just my quick ramblings. By the way, the note about the 10% above keeps conversations going in my AI and Robotics groups for hours. You have to keep religion out, otherwise its too easy to say "because <insert> says so".