becasue in this sentence the "have" is an infinitive, not a conjugated verb She doesn't swim ( do is the main verb (the conjugated one) and "swim" is an infinitive) She doesn't eat well (eat is infinitve ) He is doesn't have his books with him (have is infinitive) or with another help verb/main verb he wants to eat icecream (eat is infinitive) she loves to write (write is infintive)