As others have noted, the 9ish - 5ish, M-F operating hours are there because it’s effectively a norm, particularly for businesses which conduct their operations with other businesses.
I’ve spent most of my career at advertising agencies; our entire business is creating work for our clients (i.e., other businesses). A significant part of that is participating in meetings with each other, and our clients, and answering phone calls and emails from our colleagues and clients. It behooves us to be available when our clients are doing business, and unless it’s a client that’s not US-based, that’s typically 9-5, M-F. (I’ve known colleagues who have worked on accounts based in Europe and Asia, and it leads to having to take meetings either very early in the morning, or very late in the evening.)
Even if your business is one that sells directly to consumers (i.e., a store, a restaurant, etc.), you need to be open and operating when most people are able to patronize you; with some exceptions, that means being open in the daytime (though it often also means being open earlier than 9 a.m, and later than 5 p.m.)
A business which has little or no direct interaction with other businesses in the course of its daily operations could, in theory, operate with whatever schedule they chose – for example, a manufacturing company which just cranks out widgets could choose to operate its production lines from 6pm until 2am., Wednesday through Sunday. At that point, the challenge is finding employees: most people have families and friends with whom they do things away from work, and they want to be able to do things (have dinner, watch TV, visit, go to sporting events, etc.) when their friends and families are available, and that usually means evenings and weekends.
I have a few friends who work second and third shift, and their social schedules wind up being very limited, because when their families and friends are awake and available, they are either at work or asleep.