Because….
They make you swear. Or affirm, but it’s the same thing. Asseres Hadibros, hello?
People dress not tznius. Shmiras eynayim, hello?
Courts ruled against G-d in schools, courthouses, and the pledge of allegiance. Kefirah, hello?
As a juror you are forced to fraternize with goyim. Am segulah, hello?
Jurors are entrusted with being the triers of fact. Only Hashem can determine the facts. Kefirah again.
Plus of course it is Bittul Torah because you can’t learn while listening to the proceedings.
But the main reason why court is no place for a frum jew is gezeirah atu coming as a defendent.