Why not wait to release them until enough data has come in that it's settled? Serious question, what's the downside?

Because the market values the early results and has an adult understanding of what the numbers mean.

Because the law requires them to release reports on certain dates, so they do so and then make corrections as more data comes in.

It’s never settled as the data is never perfectly accurate or exhaustive. We just have to do with the caveats and understand that perfection does not exist, even if you have more data than you can handle. That’s what error bars, uncertainty analysis, and confidence intervals are for.