Question

why do oxides of non metals when dissolved in water give acids and oxyacids.

Answer 1

That is because of their low I.E, high Electronegativity and high electron affinity,
so when non metal oxides are added to water they form a strong acid or Oxyacids. Making the mixture over-all acidic.

That is,
non-metallic oxides dissociate into their constituent ions in water, they give H+ ions in their aqueous solutions forming acids and oxyacids.

For example:
CO2+H2O⟶H2CO3
SO3+H2O⟶H2SO4