Originally, bleachers were simply made from 2x4 remnants used for seating the nobility. Slaves held up the boards 18-24" from the ground, allowing the upper classes amphitheater-like seating without the need for a massive construction project. While the nobles were able to rest comfortably on their rears, no such relief was available to the slaves, who were, indeed, standing for as long as necessary.
Romans disliked referring to these temporary arrangements by their formal name, "seating on spare boards of wood held by standing slaves." The name was since shortened to, "stands."