For one thing, the G19 came out several years before the G26. Secondly, the G26 frame is too short to accommodate a rail mounted accessory, such as a light. Many use the G19 as a duty piece and like that option.
Does this hangup of yours extend to cars? Would you not buy an SUV that's built on that same manufacturer's truck frame?
As someone else said, streamlining production = cost savings to us. Compatibility is a feature, not a problem.