Technically life is also optional, just throw a toaster in the tub and get it over with.

But it's kind of a moot point to make because most of us still feel like we have some goal in life that we aspire towards. Similarly most games also have a goal and if you want to reach that defined ending then there are certain parts of the game that you have to complete, hence them being mandatory. When it comes to bosses like the Nameless king beating him isn't a requirement to finishing the game (compare to the soul of cinder whom you have to beat to get the endings), thus the boss is not mandatory and is instead optional.