Because it is derived from the long extinct rank Sergeant Major General which was below Lieutenant General. They simply just took the Sergeant off because it is an enlisted title now.
Blame it on the British military: While majors outrank lieutenants, lieutenant generals outrank major generals. This comes from British tradition: Generals were appointed for campaigns and often called "captain generals." Their assistants were, naturally, "lieutenant generals." At the same time, the chief administrative officer was the "sergeant major general." Somewhere along the way, "sergeant" was dropped.
This has troubled me for years too. I have been trying to have this corrected since I was around ten in 1964. As a young student of the military I realized that a Major General should out rank a Lieutenant General. We have Lieutenant Colonel, why not create Lieutenant Major's. I am so busy that I forgot about trying to change this mistake. Contact me if you are interested in righting this wrong. How about this. One star for Lieutenant General, 2 stars for a Major General, 3 stars for a Colonel General, and 4 for a General and 5 stars for a General of the Army. Now replace the stars with hearts and what do you have? May-be Lieutenant General, Captain General, then Major General. What do you think soldier? OUT
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Yeah I agree just keep it how it is. The same reasons why sergeant isn't spelled "sargeant" and my colonel isn't spelled "kernel" that's just the way it is.