As you know the Imperfect tense is used to express a repeated action in the past, like the sentence you mentioned. I have changed the translation in English to "used to sing" so it is more clear.
As you are saying, why not cantó?
If you see this sentence in English, "She sang in a band during high school", despite having a timeframe that shows the action is a completed action (she is not in high school any more), this can take both the Imperfect and the Indefinido. If you use the imperfect: cantaba, the speaker is seeing "her" singing in the band during her time in high school, with no specific beginning or end. If you use the Indefinido: cantó, the speaker is seeing the action of singing as an action with a clear beginning and an end.
We appreciate the difference between the Imperfecto and the Indefinido is quite tricky. It needs a lot of practice.
Here is a lesson about this. It is explaining the general difference between these two tenses
Using El Imperfecto vs El indefinido (general use)
I hope this clarifies your doubts.
Similar confusion arose with "He trained every day during the competiion. The competion is over. The training period is ended and is a completed series of actions in the past. so preterite would seem appropriate. The question concerning singing has been changed to "used to sing" to make it clearer. Please change "trained" to " Used to train"
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