Because some manufacturers don't know shit about nutrition
There are plenty of studies about calcium limiting Zn absorption
Here is one of them :
Too much calcium, especially from a non-soluble/non-absorbable source, can be responsible for kidney stones, bone spurs, urinary calculi, suppressed bone remodeling and mineral imbalances. In a study conducted in a group of men and women (aged 21-69 years), researchers measured the acute effect of a 600 mg calcium supplement on zinc absorption from a single test meal supplying 7.3 mg zinc. Zinc absorption was reduced significantly by 50% when the calcium supplement was given with the meal (Wood RI, Zheng JJ., High dietary calcium intakes reduce zinc absorption and balance in humans. Am J Clin Nutr 1997 Jun; 65(6):1803-1809). High intakes of calcium can inhibit iron absorption if both are present in the same meal (Whiting SJ. The inhibitory effect of dietary calcium on iron bioavailability: a cause for concern? Nutr Rev 1995 Mar; 53(3):77-80).
And you don't need to WORK with kids to need extra immunity help...just having one who comes home from school everyday is enough to ensure you get exposed to all the creepy crawlies!