Because that is the way it evolved.
The nautilus shell is a fantastic example of sacred geometry.
Nautilus pompilius
The only extant cephalopod that produces an external shell is the chambered nautilus. The shells produced by squid and cuttlefish are internal.
Nautilus
Its' shell.
with its shell
while the chambered nautilus for buoyancy on the ancestral mollusc's shell.
The Nautilus tattoos symbolize the divine proportion. The number PHI repeats throughout nature like in the proportions of the nautilus shell.
it has long tentacles and a stern shell
The Nautilus.
A chambered shell
No. It is a cephalopod that lives in a spirally chambered shell.