Why do the cells produced from meiosis have half the number of chromosomes that mitosis produces?

Meiosis occurs almost strictly during ual reproduction. The reason that the number of chromosomes and halved is to allow the union of half the gametes(chromosomes) of one donor to combine with half the gametes of the other donor (example man = 1/2 gametes, women - 1/2 gametes). This allows for genetic diversity. In essence the "loss" of chromosomes are replaced my the gametes of the opposite donor. If the chromosomes were simply doubled as in mitosis, you would have a clone since you would be only doubling the existing chromosomes.