AT&T rolls out Android 9.0 Pie for Samsung Galaxy Tab A 8.0 (2018)
The reason is that these important updates must be tested first before they are released to the public. The Galaxy Tab A 8.0 (2018) is a relatively new tablet, having been on the market for almost a year, so it's perfectly eligible for an Android 9.0 Pie update.
Unfortunately, it took AT&T until today to finally release the long-awaited Android 9.0 Pie update to the Samsung Galaxy Tab A 8.0 (2018). In case you still own the tablet, you'll be happy to know that AT&T has just confirmed on its support website that the Galaxy Tab A 8.0 (2018) is getting updated to Android 9.0 Pie.
The update weighs about 1.1GB and includes the August security patch, but aside from that, there are no other mentions in the changelog posted by AT&T.