Because there's such a large corpus of 5000 year old cuneiforms waiting to be added to the training set? Ok, that sounds a bit snarky, but how would you expect to improve certainty when there is so little data?
"There are far more of these tablets in existence than I could have imagined. In fact, between half a million and two million cuneiform tablets are estimated to have been excavated in modern times, of which only approximately 30,000–100,000 have been read or published."
https://library.stanford.edu/blogs/special-collections-unbou....
Right now, Assyriologists will prioritize - if the tablets come from, say, someone's old garage, they are unlikely to be translated. If it comes from a palace or a temple, then the odds are better. But imagine the situation where someone ended up with a copy of the Declaration of Independence in their garage.