why benzoic acid is stronger than ethanoic acid?

Benzoic acid is the stronger acid compared to ethanoic acid.

Acidic means - it depends H+ dissociation and the formation of conjugate anion.

C6H5-COOH + H2O -> C6H5-COO- + H3O+

CH3COOH + H2O -> CH3COO- + H3O+

Benzene ring is electron withdrawing and a methyl group is electron releasing.  The benzoate ion resulted from dissociation of benzoic acid is stabilized by resonance while the acetate ion resulted from dissociation of acetic acid is not stabilized. The electron withdrawing power makes the releasing of the H+ ion easier in benzoic acid compared with ethanoic acid.