Why aren't methanal and ethanal also ketones? Why aren't the aldehydes methanal/formaldehyde and ethanal/acetaldehyde also ketones? Ketones are defined by replacing any H - C - H in an alkane with C=O, which is true for both of these: methanal replaces two hydrogen atoms in methane with C=O, while ethanal does it with ethane.
Aldehydes deserve a different name because they have different properties, for example being easily oxidized to acids, as in the Benedict test which distinguishes "reducing" sugars like glucose (an aldehyde) from non-reducing ones like fructose (a ketone).  The relation between aldehydes and ketones is like the relation between alcohols and ethers, both of which can be described as R-O-R, but the properties are significantly different when one of the R's is a hydrogen.