Because electricity follows the path of least resistance. The button pin passes through a collar in the top of the switch body, before touching the bottom of the battery. This collar is where the main electrical and physical contact is. The spring where it is in contact with the button offers a fraction of this surface area. The spring is in contact with the switch body, but this isn't the path the electricity follows.
Maybe I am Mistaken, but as I understand it, the Flow of Electrons is Directly proportional to the Amount of Resistance of a Conductor(s).
So if I have a Dual coil Atomizer, and if One Coil has .5 Ohm Resistance and the other Coil has 2.0 Ohm Resistance, there will be a Flow of Electrons thru Both Coil. Not just thru the .5 Ohm Coil.
Albeit, it is a "Easier" path for Most of the Electrons to take thru the Negative Contact of Mech Mod Switch. But if the Spring is in Contact with the Switch Body, Isn't there an Flow of Electron thru the Spring?