For example- Does she look smart? Did she come yesterday?
That's because the most important aspect of a verb (or any other distinct meaningful element of speech, for that matter) is its semantics—what it actually means. The base form of a verb (its most rudimentary form) has got exactly that and nothing else. Other aspects of a verb, such as its tense and mood, are taken care of by the auxiliary verb—in your case, does and did.