Why is training regarding the loss of RAIM given so much more emphasis than training regarding the loss of SBAS?
From a safety perspective RAIM is more critical than SBAS.
If you are flying using GPS, you always rely on RAIM to keep you safe, sometimes you need SBAS too.
Receiver Autonomous Integrity Monitor is a function within an aviation grade GPS receiver that monitors the integrity of the GPS system. Without RAIM, a malfunction in the GPS system can cause large position errors that go unnoticed. These position errors can lead to nuisances such as controlled flight into terrain that really ruin the day.
SBAS on the other hand is (merely) an augmentation to GPS. It improves the vertical accuracy by a few feet, improves the RAIM function so that it is quicker to detect position errors and can detect smaller position errors. But basically, GPS without SBAS is safely usable for the vast majority of applications.
So, if you loose SBAS, you may not be able to perform certain approaches or use certain routes, but RAIM is still there guarding the system integrity.
But if you loose RAIM, the GPS position shown may be correct, or it may be 5 miles off, or 10 miles, or more; it's anybody's guess. The GPS receiver simply has no way to determine whether the calculated position is correct if RAIM is not available.

Now to your questions:
1) Why is Garmin so indirect about the loss of SBAS in their annunciations?
I can't speak for Garmin as to why they emphasize RAIM more than SBAS in their annunciations.
In case of case of the SBAS loss, the annunciation focuses on the operational effect (Approach Downgrade). In case of the RAIM loss, the annunciation focuses on the cause / technical description (RAIM UNAVAIL and LOI).
My guess is that SBAS was added later to their product line as RAIM, and their interface design philosophy had changed from focusing on the technical cause to focusing on the operational effect. For system commonality reasons they did not update their pre-existing RAIM annunciations when adding SBAS.
2) Why do pilots not also perform SBAS predictions like they do for RAIM?
SBAS depends mainly on the availability of a healthy SBAS satellite. The SBAS satellites are predicted to be healthy, and they cover a fixed area of airspace (they are geostationary satellites). The availability status of SBAS in not dynamic; SBAS is predicted to be available.
RAIM depends mainly on the number of GPS satellites visible from the GPS receiver, and how the GPS satellites are distributed over the sky (from the GPS receiver's viewpoint). This is very dynamic, depends on the  total number of GPS satellites in orbit, their health status, their distribution in orbit, the position of the GPS receiver etc. etc. For RAIM to work, the GPS receiver needs at least GPS 5 satellites to be visible (a few degrees above the horizon) and they need to be nicely distributed over the sky. So for RAIM, the situation is very dynamic, but it is predictable. That is why RAIM prediction is done.
3) Why is the loss of RAIM given so much more emphasis in training than the loss of SBAS?
Because, if not properly handled, the loss of RAIM has a much higher probability of killing you than the loss of SBAS.