Because when properly written, Hulk is immortal, drawing his sustenance directly from the universe. Banner's influence should be the only reason Hulk believes he needs to eat, drink and breathe.
Hulk 101. Hulk is "Bruce Banner's" curse or burden to bear, therefore like any good curse, it's long suffering.
Also, this better be Bruce Banner. Even though in the story he's described as Thanos' dog, I'm sick of all these "Hulk-like" gamma irradiated beings. There should be very few and each should have been a unique creature.
It is Bruce Banner, fret not. And I believe Hulk is more than a pet. He's more than likely King Thanos own personal guardian protector/WMD.