Ever seen an arc welding in action? Once the arc jumps onto the grounded plate, you don't have much control left on it and in a few miliseconds, the surface will be blazed, and the electrode is introduced to this hot spot, instantly melting it onto the surface, creating atomic level bonds.
Using an arc welder, you would have no control of the temperature, and it would probly boil battery in a few miliseconds.
Spot welding, works, because there is much less energy flowing through the electrodes, in a much smaller area, and by applying pressure onto the spot welding tab, you reduce the amount of energy needed to form bonds by placing the parts that are being welded together, in the end, you get the same welding action, but before the battery plate can even heat up, the process is done.
just my two kuruÅŸ.