Here's why:
* Stem Changes: The stem of the verb changes in certain conjugations. For example, the stem changes from "sort-" to "sort-" in the present tense ("je sors, tu sors") and to "sorti-" in the past participle ("sorti").
* Irregular Forms: Some of the conjugated forms are irregular and don't follow the typical patterns of regular verbs. For instance, the present tense form "je sors" is not "je sortie" as you would expect from a regular verb.
Therefore, sortir needs to be memorized as an irregular verb with its specific conjugations.