Why does the presence of extra sex chromosomes have a much weaker effect on the patient’s phenotype than trisomy for autosomes?

1) Sex chromosomes encode relatively few features, because their main task is sex determination. Somatic chromosomes contain many genes that strongly affect the phenotype, especially when their number increases.

2) In cells, extra X chromosomes are inactivated, and only one remains working, respectively, the effect of extra chromosomes practically disappears. And the Y chromosome contains very few genes. Therefore, extra sex chromosomes can affect the phenotype, but do not lead to fetal death.