mg
Gee, wouldn't it be a good idea for a supreme court judge to have judicial experience? mg

Not necessarily. If it were, I'd think the Constitution would have specified that.
I think they all have to be lawyers (or at least have been), but there are many other perspectives which are valuable to have represented on the bench: prosecutors, defense attorneys, legislative experience, labor, management, etc.
It's not that unusual to have a SCOTUS justice who had never been a judge. At least 40 past SCOTUS appointees have not had previous experience as judges.