Because it's part of the day to day upkeep of life in general. You don't get paid for cooking, cleaning, or doing laundry (unless you're a middle class child earning pocket money from your parents). It's just stuff that has to be done.

Part of daily upkeep if it's just for yourself (possibly plus partner/roommate/whatever). I think we're talking about cooking/washing/whatever for a couple hundred people. As a job.

You also dont get to go to the toilet in, be confined in a small room, barred from using the internet and generally deprived of your freedom in life in general.

This is true, but that's the punishment that's been handed down having been convicted of a crime.

Not being paid for things that you're otherwise also not paid for isn't a punishment, that's just normal every day life in general.