Because it is a TV program and it makes for more interesting viewing.
The reality of a closed prison ( which is most of them) is that the inmates are locked in their cells for 23 hours a day. You only get out to collect your meals and have to return to your cell to eat them.
Toilets are in the cell and you have to use the loo in front of your other cell mates , up to 4 in a cell made for 1 !
That makes a very poor and uninteresting program!
The stuff you see on TV and films where the inmates sit in a canteen to talk and eat their food with the other prisoners is just a story.
Open prisons are better , but very few prisoners qualify to be sent to open conditions!