formula of water is H2O why cannot we write as HO2? please explain?

Because water molecule is made of 2 hydrogen and 1 oxygen. So we write it as H₂O.
HO₂would mean there are 2 oxygen and one hydrogen, and this is not the case.

And we dont write H₂0 as OH₂because we write the element with a positive oxidation number first and then the element with negative oxidation stateIn H2O.

Oxidation state(o.s) of H is+1 and that of O is -2.