Soldering dump a ton of heat into the cell, which has chances of destroying the cell. That's why most of the cells are spot-welded: its similar to soldering, but its much quicker and is localized only on the part of the metal that need to be melted, so the heat don't have time to reach the cell itself.

Taking a guess, probably has something to do with batteries heating up and solder having a low melting point.