Because Draco is still in the (childish) mindset that if you make efforts to avoid the bad things, then the bad things won't happen to you. If you're pretty, clever, talented, and rich enough, you can always make them disappear. So he thinks Harry has somehow forfeited his immunity to horrible things because a horrible thing happened to him, and he resents Harry for wanting to make Draco live with a horrible thing.

Lucius and Narcissa are at the Manor. Draco and Scorpius are here because Draco wants to show he's a patron of new things by setting his new house up with new magic (and then he'll hold charities there).

Thank you!