It makes you wonder why sinkholes don't appear in funeral parks if a biodegradable coffin were used. When it collapses, the ground above would sink. Also, runoff water from a funeral park goes into the groundwater system. In southern cities, this means peoples' wells will be sucking up funeral water. If you live downslope from where Michael Jackson is buried, perhaps you can suck in some of his former molecules. *ahem*
I recall watching a documentary a few years ago that talked about eco-friendly burials. This one place they went to just wrapped you in some sort of quickly biodegrading fabric and buried you with with only a stone or small wooden cross as a marker. Out of curiosity, I did a quick Google search about it and there are a handful of other similar options, if you so care to check it out.
Point was that you dont need an actual coffin. I would say you dont even need the fabric but if they choose to do it as a ceremony, family likely doesn't want to see your rotting corpse since these options also tend to omit any embalming process, at least chemically.
Reminds me, another fun alternative would be a funeral pyre. Who doesn't love a good bonfire with food and drinks? Not the most environmentally friendly option with the whole burning aspect, but still.