Why use the terms diploid and haploid instead of discussing specific Numbers of chromosomes?

because number of chromosomes in each organism is different
example humans have 46 whereas tigers have 38

Because diploid and haploid don't just mean numbers of chromosomes; they describe whether chromosomes occur in multiple couplings.

For example, you could have two organisms with 46 chromosomes, but one could be haploid and another could be diploid. This difference would make them very different organisms. The haploid organism will have 46 chromosomes that are completely independent; they all carry different genes. The diploid organism has 23 pairs of chromosomes, and the pairs will carry identical (or almost identical) gene sections; if a gene is found on a part of one chromosome, a similar gene is found on the corresponding part of the other chromosome.