Why is the reaction SO2 + H2O → H2SO2 not balanced?

There are more oxygen atoms in the reactants while there are less oxygen atoms in the product.

Both sides of the equation is supposed to be balanced for a balanced equation. If any one of them isn't balanced, the equation remains unbalanced.

The main reason why the reaction above can not be balanced is:

This chemical reaction SO2 + H2O -> H2SO2 is not correctly written.

It must be: SO2 + H2O -> H2SO3

hope this helps