Can't they just.... ahh forget that, i don't know again where the author want to bring this. Well, 2million copies, who in the world don't want to continue that for longer ?
Her reason...:
A: Doesn't want her grandma's condition to get worst.
B: She's used to act as his (rental)girlfriend and doesn't want it to end. Love? Easy money?
This right here is why this manga is so popular and yet so many people hate it. It builds up cheap tension and hooks you in, but with no reward. You expect things to develop and bam!, grandma is just fine, fuck you very much.