There are large error bars right now. The percentages will likely increase and increase as the error bars reduce and the earth takes up a larger and larger space in the potential zone it can pass through.

Up until the earth is outside that zone and the chance drops to zero. So expect the "percentage chance" to keep rising until they have enough to data to rule it out entirely.

Why can't they get all that over with now and say what they think the actual chance is?

I think you'll find that what's been said is the best current knowledge.

Why can't astronomy be a more exact science? Well, how much are you willing to contribute to launching more space telescopes for this? And is the budget for NASA currently increasing or not? ( https://news.ycombinator.com/item?id=43090862 )

Cities on the impact path could be asked for funding.