Independence and self respect is sexy, and doing chores is a form of independence (you know how take of yourself) and is a form of self respect (you don't let yourself live in filth). It also implies that you don't view your gf as the designated house maid and not being humiliated/burdened like that is a big relief, especially for women who grew up around sexism. Who might have internalized the idea that men doing chores is on par with something like gift giving or going on dates, something special done out of love, and not just something that needs to be done. She could genuinely find you doing chores sexy, but also having to do less chores is a bonus no matter who you are, it can be both.