Why are sperm and egg haploid cells?
Sperm and egg undergo fusion to a zygote. The zygote will undergo developmental stages and growth to eventually form an individual of a specie.
The number of chromosomes must be constant in a specie. To maintain a constant number of chromosomes in species the gamete cells undergo meiosis. Meiosis is the reduction division because it reduces the number of chromosome. It only occurs in diploid cells and reduces the diploid cells (2n) to haploid cells (n) eg: gamete cells.
Both gametes after meiosis will have half the number of chromosomes and when fusion occurs the original chromosome number is restored ensuring constant number of chromosomes in species.
To simply say it if gametes are not haploid the same chromosome number in species will not be retained.