Why don't we do something about it? Well, I have two responses.
First. My students who do exams (very few, as they take my advice aka, they're ######) are not learning the pieces with background rhythms, so when they do the exam, they are at ease without a rhtyhm. If there was suddenly a double bass bopping away with a brush kit, I think it might throw them. However, in counter-argument to my own sentence, yes, it would be good to have the LESSONS, and EVERY lesson with backing (decent backing), but my argument against that is, the backings are too artificial. I'm sorry, but my stance on Jazz is extremely set in the sense that I respect it. I worship it. It's mental stability to me. Now alot of people would't understand that, but thats just my level. I dig it so much, and listen so much to the changes and melodies and the bass, the drum, the intonations.. I listen for hours a day, to so many recordings, and I have now got a stage where I respect it so much and understand it so well that I take offense when people (not that they are here im just on a roll) badmouth it, or say its easy, or give weak-formed comments about how to play it, when really its an incredible art to master. So, in conculsion of this first point, live backings in an exam would have to be suited so well to the chords being used (should they be enhanced from the original score), there timing would have to be so tight etc, that kind of situation is beyong grade 1 2 + anyway! And second, I don't disagree that having backings in Lessons would not help, but I have had no problems with students having no backing, lessons or exams, and still get distinctions at any level.
Second. It's not hard to find a drummer and a bass player. There are things called directories, and if my student wanted to pay extra for two live band members to learn the peice (ok, it might take 10 minutes but thats Jazz musicians

) and cost about 100 quid but is it worth it?
Thinking further about this, it would be good if the ABRSM had solo and band jazz exams. That way, students specifically learn how to deal with a Band environment, and learn to share 8s, and work along the bass player etc, and lessons could well be with a band rehersal, or the band record the backing track themselves and that is used in the lessons so it's less artificial.
Interesting babble though I think.