Depending on the viewer and their ability to suspend disbelief or logic, YES there may need to be a why!Does there have to be a reason? Adam Sandler's acting and film quality took a huge dip into the outhouse a long time ago. Netflix says his movies are some of the highest watched on their platform. This isn't me bashing people who like dumb movies (I like some myself), but people want some entertainment at times and not a serious, to the point, art house film.
With sci-fi I’m open to unexplained things, but in this case, they went to a parallel universe were all indications were that biology and physics were basically the same. So an animated detached arm with knowledge to share and a desire to write a message for the benefit of the crew was hard to suspend.