These commenters are cancerous. Not as cancerous as this contrived melodramatic bullshit plot development though. We know for a fact that she not only cares for him as a close friend but also has romantic feelings for him, so her reason for doing this is....?
@DarkLordAlfie Because she think that there's no future for them. She's being the big actress would sacrifice much of her career if she choose to be with him. And also she knew that he's in love with the Idol not the otaku girl she is. So she chased him away to be "normal" with a "normal" girlfriend.
Imo, the manga should end with him realizing about the different of loving another real person and loving the idea of that person in his head. Mai did also love the idea of brave MC. An epilogue where they go back to being friend and slowly get to know each other would be great. But i don't think that's the case here.