A better question is: why do we actively discourage single parents from marrying and working full time? In many places, full time employment with low skills is a ticket to disaster.
Let's say that you did not have to work. Imagine being free for decades. What do you think you would actually do for all of that time?
Most people do seem to need to decompress by doing nothing useful for a time; I find a month per year of work turns out to be about right. So let's say you do whatever you need to do to recover (games, videos, books, ...). What can you imagine doing after all that escapism becomes boring? It may not be employed by a boss, but would it be a useful contribution to society?