It is a convention for video games made in Japan, when loading data, to display the legend
Whereas titles made overseas generally stick with the more succinct
Is there any linguistic reason for the addition of the word "now"?
I doubt there is a good linguistic reason for this. Probably someone happened to come up with "Now Loading", and other developers followed without thinking much about it. This is not surprising at all. After all, Japanese people have coined tons of wasei-eigo terms and kept using them without thinking about them.
Another similar example is "Sorry, this site is Japanese only"; around 2000, many Japanese people somehow believed this was a polite and natural English expression, and put this sentence at the header or footer of their websites. No one knows who started using this, but it did spread like an infectious disease.
Whether you are asking about games that are in Japanese or in English, I would assume (pure speculation here) that, as the game was likely made for domestic consumers originally or by a Japanese team, 'Now Loading' was easier for non-English speakers to comprehend. When overseas games shortened the legend the Japanese likely saw no need to do so, as there is really no compelling reason to do so. Also, their Japanese customers are used to seeing it displayed in its entirety.
That being said, I have not noticed this particular difference that you have.