Why is disintegrate force damage?

My question is simple, why not necrotic damage? Since necrotic is about death and decay, it feels more fitting. Sure is sounds reasonable, you use fore to pull apart atoms, but then it gets weird, like its a single point, creature, or thing, so no explosion. Also it always converts it to ash. And finally, it doesn't move them, I'd imagine it's supposed to be a large beam that "rubs them out of existence", but in practice, it doesn't feel like that.

Force damage in this game is not at all related to pushing or pulling. It's not very well named at all really. From my reading of the various spells and effects that use it, I would define Force damage as a direct, violent attack on the soul or life-force of a creature - this being why almost nothing has resistance or immunity to it.

Necrotic damage might also attack the soul, but it is more a cold, draining, deathly attack. Radiant likewise, but hot and divine. Force feels more primal.

Disintegration is not trying to drain your life force like a vampiric touch, rather it is ripping apart the animating life energy of a being, and then the body falls to ash as a result.

That's just my way of thinking about it anyway. The real answer is it uses that damage type because the book says so... 

Force damage is basically just pure damage, and Disintegrate just destroys you, so Force is definitely the best fit.

necrotic damage is like your skin rots. The skin is dried up, shriveled, and mummified, but the skin is still there.

force damage, at least to me, always seemed like "destruction at the atomic level", like the atoms of your skin are turned to a gaseous vapor and float away.

I have always imagined Force damage to basically be non-specific magic damage. It isn't elemental like fire or lightning, and neither death and decay nor radiation like necrotic and radiant. It's just a pure magical force.

Yeah, force damage is defined somewhere as pure magical force. It's the damage type you use when you're just doing damage.

Its main property is that nothing resists it. (Or almost nothing.) Which is appropriate for disintegrate. There's no reason the undead would be resistant to being poofed.

That is an interesting way to think about it, and would make sense with eldritch blast. I just looked through the spells that use force damage, which is far less than I thought, and I think your right, they seem to use Force damage as Life Force, which if that is the case, they should really rename it.

Think of Necrotic damage as degradation, Disintegrate is turning your body from biology to physics. 

Disintegrate doesn't involve the rotting away of death and decay. Disintegrate is the breaking of bonds at the atomic level. 

What the Ark-ghosts did to the Nazi soldiers in "Raiders of the Lost Ark" was necrotic damage. What Dr. Manhattan did to Rorschach in "Watchmen" was force damage. 

Force is basically kinetic energy without mass.

Also, Disintegrate has always been a highly effective tool against undead, making it necrotic damage would mean that a lot of undead creatures were resistant or immune to it.