Why does test optional exist

Genuinely I don’t understand why test optional exists. For reference, I’m a junior with a 1560 sat and I have a 3.5 uw. I go to a Bay Area public hs (the stupid competitive ones) and it’s not like I don’t try hard to get A’s I just kind of suck at school. So why should my qualitative data representing “how smart I am” be an arbitrary number that varies between schools? How is that fair. It’s not even a question of resources, atleast not for me. I studied using only Khan Academy and free online etymology classes 10 minutes a day from start of sophomore year to now. And I have to work 4x a week to support myself, because truly anyone can fit that in their schedule, why do people whine about standardized test so much??