Question

why are ice cream and steam not called allotropes of water

Answer 1

Allotropes are different forms of an element and they differ in the way the atoms are bonded. 
Diamond and graphite are allotropes of carbon.  In diamond each carbon is bonded to 4 other carbon atoms while in graphite each carbon is bonded to 3 other carbon atoms.
The bonding between hydrogen and oxygen in ice and steam are the same. These two only differ in the state, ice is solid while steam is a gas.

Answer 2

the phenomena of allotropes is only observed in elements, such as carbon.
but steam[H2O] and ice cream[whatever the formula is] are both compouds.

so they are not called as the allotropes of water.

thank you