Is there a way to change that and if so, why is it the default to just have it floating above the void instead of on the nearest solid block?

Finding the nearest solid block can be heavy on resources (depending on how far away the nearest solid block is). Also I think it makes sense to generate the grave as close to where you died as possible.
There is a config though that you can enable that will cause the grave to be generated at the last block you stood on. I would assume you'd be happy with this?