There is a positive command to remove all hametz. Gebroks is just going out of your way to do so. It’s like wearing a really nice set of clothes on Shabbos (one only has to wear one’s best clothes, and they have to be respectable – going out of the way to own a really fancy Shabbos suit is a hiddur, not a humrah).
If it was a humrah, such as kitniyos, it would be prohibited on the 8th day.