Porque eles ignoraram detalhes do livro.
In the book he has two duplicate rovers. He uses the second one for spare parts and as a trailer that is towed by the first rover (in the movie he just uses a flatbed trailer). The second rover has to carry life support equipment that doesn't fit, so he has to cut it open. - reddit.com
Eu não li o livro, mas todas as fontes disseram o mesmo. De Questão semelhante de Scifi.SE:
From Chapter 20 of the book:
LOG ENTRY: SOL 376
I’m finally done with the rover modifications!
The tricky part was figuring out how to maintain life support. Everything else was just work. A lot of work.
I haven’t been good at keeping the log up to date, so here’s a recap:
First I had to finish drilling holes with the Pathfinder-murderin’ drill. Then I chiseled out a billion little chunks between the holes. Okay, it was 759 but it felt like a billion.
Then I had one big hole in the trailer. I filed down the edges to keep them from being too sharp.
Remember the pop-tents? I cut the bottom out of one and the remaining canvas was the right size and shape. I used seal-strips to attach it to the inside of the trailer. After pressurizing and sealing up leaks as I found them, I had a nice big balloon bulging out of the trailer. The pressurized area is easily big enough to fit the oxygenator and atmospheric regulator.
Então, a cena parcialmente adaptada do livro, não faz muito sentido.