Because you don't modify the genetics of terminal, living adults, you want to modify the genetics of single-cell healthy zygotes (who will, you hope, later develop into healthy adults).

Sure you do. A lot of the promise of CRISPR therapies were for modifying cells in an adult or child. Using CRISPR/cas9 to “fix” a genetic issue in someone who was already born. For example for adding back a good copy of the gene that causes cistic fibrosis or sickle cell anemia. There were also potential cancer therapies based on the technique.

In these cases, the presence of cas9 antibodies is a severe limit to many of the techniques that these companies were working towards.

I agree. My father has terminal cancer and has been practically banging down doors to try to get onto medical trials for new experimental treatments. If other people in his position want to do the same thing then who are we to judge?