Well .. now... there is this manga and now anime series about Tsukuna - the four hands curse overlord from 1000 years ago in Japan who ate humans for sustenance, ate curses for increasing his power and dominated. Now in present day Japan, that curse overlord can posess the body of anyone who is willingly eating part of his body, however shriveled and disgusting that might be - he comes back and unless there is some unexplained power in the soul or body of the person he posesses, he could stay back permanently.
The similarities do not end there - if the body of the host dies, so does the curse. If MC dies, so does Hollow and Hollow does not want that - MC has been killed or dead many times for the duration of his life before and during game time, yet Hollow has not been able to transcend - so Hollow needs to take over the body, MC willingly doing that, with MC also unwilling to take his body back, as MC has always been shown able to get back to his senses.
On that note, Hollow is waiting on some happening during the life of MC that would give him permanent domain over the body of MC, that event has not happened yet.
Hollow could only get acess to the material plane with the thinning of the veils between dymensions, but Hollow is not a schollar in al things occult, not like the witches who have been studying MC and Hollow for many years,
so there is no expectation about a behemoth wanting to end all of creation being knowleageable or dilligent in dishing out his help or diss to MC - Hollow has been shown to either black out or just enjoy the show, watching MC suffer, so Hollow being contempt with endless suffering is right on course with how he has been behaving as of late in this game.
