i think it IS because they think it sounds cooler.
and in fact, its not exactly english.
some of it are engrish (improper english)... haha.
Edit: and english is a global language & quite popular in japan, i heard (but still, the main reason is because it sounds cool)
here's a video of a japanese band who sings english not engrish!
but still maybe if its LIVE it might be different. but i couldnt hear engrish in this song. i was so amazed, couldnt believe the one singing is japanese. o.o
and koreans also often use english/engrish.
one of the good ones who sings english instead of engrish would be Epik High (check out their songs at the korean music forum) =]