That's like asking why a shark would ever attack a human even it has more food than it could ever eat underwater. You look like prey or you look like something interesting and the only way they can find out anything about you and what you're made of is a quick nip. That's why a sea monster would...
This chapter: You need to get out of our house
100% The next chapter: You're too constrained here and could change the world out there
I hate that so many manga do this weak cliche cliffhanger on the level of clickbait
Still a great story though