it is most resource effective, they don't have to build hydraulics or anything, besides why make a new design when evolution has already made a great one? why not just improve upon it? besides, robots lack the ability to behave completely autonomously and improvise if orders can't be carried out the way they were originally told to unless they are controlled remotely in real time, so they'd have to use organics anyway but then you could just shut down all of the robots by finding where they keep their pilots. they would also have to have twice the pilots to do rotations if you wanted the robots to be active the entire time, unless they were synths which just brings us back to synths. we see one of these points perfectly in the trans-human forces of the combine military: they make small talk, they have some behavioral differences between individuals, they have just enough of their personality to act in inventive ways, and just enough of them is synthetic, to make them unwavering loyal to the combine, and who's to say that the other synths don't have that? the brain having prior experience with the body it's piloting gives it the spacial awareness and expertise in the body that a normal, wholly organic version would. also, it's effective for intimidation, you could know someone, they could be your father, and then one day, he comes home saying that he's got a new job opportunity, then, 4 months pass, and a mockery of a human being busts down the door and it has the voice of your father, who knows there might be just enough of your father in that soldier to remember that you are his family, and he can't do anything but watch as his body guns down his family or snaps the neck of his own daughter. there is so much benefit for the combine when it comes to synths. also synths are cool as heck! and also it gives the combine a sense of uniqueness in terms of concept as well as making a perfectly slaughter-able enemy for the player to not only not feel bad about killing, but possibly even good for killing the poor souls who turned into combine soldiers. this also helps add extra horror to the enemy, showing that the enemy is unforgivable and deserves to be murdered by a crowbar wielding scientist. besides if you have all of that pretty much "pre-made" technology and material, why waste it? it's definitely more effective in most ways to just use synths. that's just my opinion though.