Because it's similar to real life and just like in rl you have to grind to get to the prize.
Just kidding there. Main reason why most of devs put the grind in it is so the story would get some meaning and "air time" and it's much easier to give the player the freedom of choice this way than through linear storytelling.
If you remove the grind you have a linear story that you'll pass through like reading a book and unless you are creating a literal visual novel you want to avoid that. All games have grind. From the AAA titles to ren'py visual novels. The difference is is the grind fun or tedious.
Now, the grinding part in my game is still a little bit tedious because there's nothing else to do in the game apart from seducing the girls. With added content and mechanics in the upcoming patches it should be more interesting.
Cheers.