Why the genome is divided into several chromosomes and not just a single big chromosome?
The leading contender of "why" in my former lab, is that if a chromosome becomes too long, a cell cannot fully isolate the chromatid to a daughter cell. Ie in a giant chromosome, the chromosome arms trail so far behind centromere, that the arms are of two sister chromosome are still touching each other even through the centromere have reached the opposite poles of a dividing cell. This prevents nuclear reformation. It would also mean, that the size of the cell will determine the chromosome maximum size.

At present this is a hypothesis. We have only just created the method to make ultra large synthetic chromosomes. I expected an answer is 3-5 years (2021-2023). However we say there is nothing special to a chromosome number.