Maybe the Attorney General could have lieutenants. But it doesn't seem quite right for the governor to have a lieutenant — unless he was fighting battles for the governor, which hasn't happened in ages. Why not Assistant Governor?
No, that implies that the assistant governor would actually "assist" the governor. With a Republican governor and a Democratic assistant governor, that would rarely happen.
So we're stuck with lieutenant governor — at least until the governor gets his way and abolishes the position. But that's not likely to happen. Too many legislators are interested in running for the position.