this is mainly a cultural problem. We have to accept that if we want to be truly sustainable we have to make every proces of our life sustainable, including death. This mean using the death in some way(as a fertilizer, for example)
It is indeed cultural. Take how India for example - a place where 20% of the world's population steeped neck-deep in traditional culture is jostling for 2% of the earth's habitable real estate - dodged this economic disaster by its own cremation culture. Lucky culture on that.
nothing a couple generations of "re-education" couldn't fix. take China for example, took less than 2 generations to go from country of ancestry worshipping to "i don't know where my great grandfather is buried/from"
I'd hypothesize a long history of relatively high population density created the cremation culture. Humans have been so successful because we adapt. Although the original motivation may be murky, I'd bet many arbitrary cultural institutions are actually adaptations.
This seems clear when you compare ancient religious texts side by side with what was known about the natural world in their respective times. Unclean animal species, menstruation rituals, etc.