Because there is no evidence that anything in the physical world happens for any other reason.
Of course there is.
Some events are caused by a sentient decision, while others are simply random events.
If I set fire to my neighbors house, the resulting fire is due to my ability to assemble the proper elements and my moral choice to do the heinous deed.
It was not due to random events.
When investigators look into the causes of house fires they look for clues that point to intent and design, using scientific forensic techniques to find that tell-tale order and signs of intent.
Have an idiot do the investigation and no, they probably wont find anything. But put a 30 year veteran of such investigations like a Fire Marshal and they probably will find such clues for many reasons.
The OP is asking which materialistic behavior causes mere atoms to combine together in such a way as to cause sentience to occur. The answer is 'None of them' as material lying on a floor does not do such things and have never been observed to do so.
The predictable non-response is to bend the behavior around on itself in a tautology; 'Sentient things are sentient things because they are sentient things' and ducks the entire question as to why.
Deterministic materialism cannot explain things that are not materialistic and sentience is not materialistic.