I don't think it's exactly dumb to have certain wishes to be carried out after we die. It's really interesting you have an ancient Egyptian themed pfp. They had incredible death rituals and beliefs about the afterlife. I think some of us maybe just have more poetic souls or- sides to us.
So, I don't really believe in an afterlife, but I like the idea of my ashes being scattered into the sea. It likely won't make any difference to me if they were flushed down the toilet! They may well end up in the same place anyway. But, the thought of being scattered into the sea is nice. I've always loved the sea.
Really though as humans, we've probably grown up with the idea that the dead need to be respected. Obviously because as a corpse, we have no say in the matter. But, just from a health point of view, there are laws and customs to avoid outbreaks of cholera, ebola etc. I expect our additional choices of what we're buried with, the manner of our disposal just build on those more necessary actions.
Part of it is for the living too. Having a grave to visit. Knowing that one grandparent is buried next to the other. I suppose we like to honour our dead to reflect what was important to them in life.