Why, in terms of atoms, are #CH_3CH_2OH# and #CH_3CHO# not isomers of each other?
Because they don't follow the definition of an isomer
Here is the Oxford Dictionary definition of an isomer
"Each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties."
An example of an isomer is
It has the same amount of C and H atoms, but they're just arranged differently.