Oda takes breaks for rest and recovery. He has shared some but limited information about his health, it's been reported he is diabetic, has gout and high blood pressure.
Togashi takes hiatuses because he has chronic, debilitating health issues that physically impair him. He literally cannot draw sitting anymore.
Mangaka wreck their health and bodies to keep up with the demands of their work.
Work which goes beyond simply drawing week to week.