Because the mounts on the front axle are differently constructed from side to side. The driver's side is cast into the pumpkin and never changes and can't be moved. The one on the passenger side is a formed unreinforced sheet metal tower with a lot of give in it and as the vehicle is used, that mount can easily be altered from the factory specs. If you look at one that has been used hard, it is very common to see the tangent of the two bends on the front side move closer together due to a slight collapsing of the mount as it was pushed forward.
The reason that the lowers vary little in length is because the mounts are more robust and tend to resist deformity a fair bit better. Regardless of the reasons, the only thing that matters is that you wind up with similar pre-load on the arms at ride height.
If you've spent anytime on Jeep boards, there are always the pet solutions for getting the bolts into the stock arms like jacking up one side, ratchet strapping one side back or forward, etc.. If the mounts were in the same spot, those solutions would never be mentioned because the bolts would just slide in when you got one side lined up. There is also the issue of slightly triangulated uppers and lowers as they came from the factory. If the axle is not perfectly centered with the frame level side to side, the triangulation dictates that no two arms can be the exact length. With the lowers pointed slightly outward at the front axle, as the axle shifts from to the side, one arm straightens out and the other gets a slight bit more angle which means the distance from the frame mount to the axle mount has changed if you want to keep the axle square.
Find it this way. Maybe.