This is about mortality, legacy, and humanity's fundamental inability to effectively deal with death.
The "junk" is memories. A lifetime of memories of people important to us - important to them - a record of their accomplishments. It was the best they could do. Not everyone gets a library wing, a mausoleum, or a wikipedia page. But for a brief moment maybe you have your stuff - and stuff triggers memories that your life actually happened and had meaning.
And then you're gone, and it all becomes a useless burden.
Yes, it's frustrating. But it's more existential than that. When you clean out your parents' junk, you have to confront with the reality that in some years someone else will be cleaning out yours. And the memories you're making of the rich and powerful experiences you had will also disappear into the aether.
That is disturbing and upsetting to most humans. I'm impressed so many commenters here can be completely detached from it.