Answers.
this is because the the set of diploid chromosomes in the somatic cells must be be examined/observed to determine the chromosomes count of an organism.
In order to carry out karyotypes, the two sets of or complete set (2n) of chromosomes in the diploid cells of an individual need to be examined under microscope, so that certain features for comparison between each chromosomes of the diploid pair, e.g length, position of centromere on one compare to another, the banding pattern etc. needed for accurate comparison can be obtained. thus with this any abnormalities e.g deletion, addition, or duplication in chromosomes number can be determined
Naturally under normal conditions, autosomal chromosomes ( all chromosomes in the body of living organisms excluding the sex chromosomes) exist in pairs in diploid organisms, thus this is a good pattern for comparison.
A haploid gametes only contain just one chromosome, thus there is no standard for comparison to observe and obtain desired features for karyotyping with haploid gametes.
