Members pay to feel good, and a gym that smells of old sweat does the opposite. Grimy benches, hair in the shower drains, a sticky stretching mat or an overflowing towel bin are the things that get mentioned in reviews and in cancellation emails.
Gyms are hard to clean because they are rarely empty. Many open before dawn and close late, some never close at all, and the busiest hours are exactly when equipment and change rooms take the most punishment. A once-a-night clean at a fixed time leaves the evening crowd walking into whatever the morning crowd left behind.
Most owners and managers end up wiping benches themselves between classes. That's time taken away from members, sales and coaching.