Because they skipped details from the book.
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
I have not read the book but all sources saying the same. From Similar question of 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
So its partially adapted scene from book, which doesn't make much sense.