According to my mum: “if you even miss a single day they throw the entire jury out and have to restart the whole court case again so that the new jurors can hear all the evidence”. I feel that would make longer cases exponentially impractical.
I can’t find anything about this on the internet, other than for someone asking this question in America.
Can confirm for the US at least, I was a backup juror a couple years ago in a rather dull civil case. It’s standard practice here to select 14 jurors so you have 12 plus two alternates. I sat and heard the whole case, then had lunch with the judge during deliberations where we chatted about how stupid the case was.
I suspect they might see it as their duty to point out legal stupidity; if only just so that the jurors are not given a bad impression of the whole legal system.