Why is 'become' used in the phrase 'she became a doctor'?

Because 'become' is used as the main verb in the sentence 'She became a doctor', indicating the action of becoming a doctor, it needs to be in its base form. In this context, 'become' expresses a past action, hence the use of the past tense. The verb 'become' is not used to convey an attitude, feeling, or state of being, so it requires the base form to accurately reflect the action in the past tense.