Mainly because people are cheaper and assemble new humans...
Any particular mechanical action by a robot can be done by a discrete mechanism. Trying to assemble all of these into a frame that has the visual appearance of a human being is not yet possible. We have all seen the mechanical clowns that are the best to date - and then there is the brain! Try to fit that in 3/4 gallon space.
I suppose they can put the brain in the basement and use WiFi?
As for sentience. We are figuring out all the ways in which human brains function - all the feedbacks, control loops etc.
Content addressable memory for example. Ask an army sergeant if 'private Smith' is present and he will shout SMITH, and Smith will say 'here'.
A machine would have to inspect every person to see if they were Smith.
Computers are around 4 gigahertz, and getting faster, in clock speed so they can work around this, but it is a barrier to sentience in a machine.
One by one they will solve all these disparate subcomponents that make up a human mind and hook them together to make a new born with a 4 Ghz clock. Unless they have pre-stuffed the memory with recollections etc Blade Runner style, they will have to educate this child and hope it is not a crazy child - no matter, in 20 years or less it will be done.
Is this 'child' a slave, property? An entity? Will it follow orders? Will it see it is alone and want fellowship of it's own kind?
A novel in the past created such a machine, and it was asked 'is there a God?' - 'There is NOW', was the answer.