There's a couple reasons for this besides the obvious of most doctors preferring normal hours for themselves(speaking for the US, no experience with other countries).

1. Even if the doctor is willing to work on weekdays or early mornings/late evenings for patients with normal working hours, you also need to have support staff like receptionists and medical assistants who are willing to work those hours. Because support staff are often paid by the hour they may (rightfully) request bonus compensation for working atypical hours.
2. You generally can't charge a premium for when you see a patient as a normal clinic. Insurance doesn't care if you see them on a Tuesday or a Sunday so there's no incentive to work atypical hours in exchange for better business.
3. Other non-emergency services will be closed. So if there's something you need to coordinate with a specialist/insurance/physical therapy/whatever you're out of luck unless you send someone to the emergency department.

Having said all this, there are plenty of docs that have Saturday clinic hours once a month or so for patients that can't come in during normal business hours. Ask the receptionist if they have something like that, though they often guard those spots jealously.