I think I finally figured it out. Why death is sad. It's not the losing of the physical presence, it's not even so much the loss of the person- it's the fact that in 20, 30, 40 years… they won't matter anymore.
Now before you grab your torches and pitchforks, let me explain. It's not that they won't matter to some people, because to those who loved that person, they'll always matter.
But try explaining that person to someone who never met them. Try to explain why you laugh at something they did all the time. Try to explain why you use certain phrases or words- it's like you're the Doctor having a laugh at a gas mask, but the only person who would understand the joke is trapped in an alternate universe.
This is why death is sad. Death is sad because when someone dies there are thousands upon thousands of connections that are severed in an instant. Suddenly inside jokes no longer have meaning. That phrase your mom taught you that was super special to her and your dad- no one but you and your dad understand it anymore.
You'll laugh at things that nobody else finds funny, cry at things that everyone finds hilarious. This is why death is sad.