Have you ever noticed ads for cemeteries? They advertise beautiful views, and a peaceful atmosphere. I really wonder why the beautiful view matters, after we are dead, we are not going to be sitting there looking at the scenery. Nevertheless, we seek out the perfect cemetery and the perfect plot location for our loved ones. Better views are, of course like any other real estate, higher priced.
Then the funeral homes have caskets with engravings, and are made of various materials including bronze and padded with colored satin that will enhance your loved one's features. These are in the price range of most new cars because our loved ones deserve the best. I am not being disrespectful when I say "What?" My mother, bless her heart was poor and would be extremely angry if she knew we paid $10,000 for her casket. Her brother insisted that our mother deserved the best. She did, but she is gone and doesn't really care at this point, so she would have been happy with a cardboard box if the choice was hers.
In my opinion, this is why the graveyards are over-crowded; there are too many fancy caskets that have to be buried in big cement liners (to protect the casket - who really cares, no one is going to be looking at it again). A note here, cremation would be the best solution to saving space, but many religions oppose it; so back to the casket solution.
On the island where I come from, they buried people vertically. This saves much space and you can get almost as many people into a family plot this way as you can if they were cremated.
You would no longer need fancy caskets, just some kind of a four foot by 3 foot box since the person would eventually crumble and be at the bottom 8 to 15 inches of the box anyway. If you are concerned with "looks" at the funeral, you could rent a fancy casket and then have the morticians dump the body in the box afterwards. I know, deaths are very traumatic and "dumping the body in a box" sounds disrespectful to the dead, but their feelings are not being hurt - they don't care and they would not want you to go in debt so they could be buried in bronze with satin sheets and padding.
I suppose this is a futile dream because the funeral homes and cemetery owners would lobby the politicians to block any space saving ideas. The politicians would side with the lobbyists and everyone would continue making a fortune from our dead. Eventually, we will run out of cemetery land because of this, but as long as there is money to be made, we need not rush into anything rash.