Stainless guide rods are less prone to breaking than are plastic ones. They also are less suseptable to deterioration from exposure to solvents. That beng said, the manufacturer either has confidence that the OEM guiderod will not fail often enough to cause economic or PR problems, or does not care - you guess which is more likely the case.
I have always been amused by the folks who insist on buying a functional item and then tarting it up with frills and changes so that it almost resembles a like item they could have purchased for less than the cost of all the modifications and replacement parts. This is not a mini-rant about folks who build to meet a specific need (race guns) or about basic fluff & buff work, or even getting rid of the finger grips and stippling the grip of a Glock, but about those who buy a RIA 1911 and send it off to have everything but the frame exchanged or fiddled with.