Because if they didn't, they might flop out and explode, destroying the cheerleader and her/his(?) squad completely
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Not all do. Look at the NFL cheerleading outfits, which are designed to accentuate them. Gymnasts do wear leotards that flatten the breasts because they don't want people to focus on the breasts but not on the routine. In most cases, this is not necessary because of the severe dieting that many gymnasts go through.
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Why do sports outfits for males include jockstraps?