Why can't they ever explain why humanity going extinct is bad?

I've so often been hit with "but then humanity goes extinct", mostly presented as something I would not have thought of myself and I should be horrified by and should cause me to instantly change my mind. I of course see no problem with voluntary human extinction but when I ask why they think it's so bad they can almost never answer. Most of the time they are astounded I would even ask that. The most I ever got was that our art would stop existing, but that really doesn't seem like a problem to me because we are the only ones that appreciate our art in the first place. Wish someone could actually explain why human extinction is such a bad idea to them.