Question from a skinny guy about fat people in the workplace

I work at a bakery/deli as a supervisor/baker. One of my employees is quite large, I'd guess about 350lbs. She's a nice person, and more or less does her job. When she can, anyway.

Lately she's been coming in with doctor's notes and requesting days off. She has bad ankle pain. She's feeling fatigued. She has bad back pain. Guess who gets to cover her shifts? I do. I'm on salary so every time I have to cover one of my employee's shifts it's just taking away from my personal time. Time I could be spending with my wife. Instead I'm working 12 hour days while my employee goes home and eats excessively.

Because she brings in doctors notes I can't fire her.

Can you please explain to me why I should "accept" this? Why should this be OK in our society? Why should healthy people have to work more to make up for people making poor life decisions?