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.
My serious answer is that people like games which “reward exploration” but you can’t have real exploration without the possibility of missing something.
You need to have optional areas to reward exploration, but that’s not the equivalent of “the whole game is optional”. Even BotW where 99% of the game is completely optional you and everyone else still have to complete the following steps: activate the first tower, clear 4 shrines, talk to the king, kill Ganon. Minecraft for example is an “everything is optional” game and plenty of people would argue it’s less of a game and more of a pure sandbox because you have to set your own goals and do your own thing and you’re done when you don’t feel like playing anymore. You can’t “finish” Minecraft.
A game designer will always need to create some mandatory parts if they want the game to have an end state beyond “you died”. Mandatory parts don’t invalidate exploration except when “everything” is mandatory.
