Why don’t segmental retaining walls need any piers or anchoring below grade

I have built two soldier pile retaining walls (where you slot the boards between I-Beams you sink into the ground) with good success – but that success was due to:

• sinking the piles at least their above ground height (ie. 5 feet above and 5 feet below)

• very generous "deadmen" going backwards under the retained material

But now … we have a much bigger wall to build – 80 feet long and running from 4 feet tall out to a corner that is 12-13 feet tall. Soil samples show 8 feet of "creeping" soils and our wall engineers tell us they want us 9 feet PAST the creeping, so a 12 foot corner requires a 12+17 (let's call it 30) foot H beam. And that's just one of them …

So that's getting really expensive and complicated and the engineering firm said to just forget the soldier pile wall and switch to a segmental retaining wall – just the plain old versa-lock pre-fab blocks that everyone uses (although we'll probably use "big blocks" that are 200-500 pounds each …

My question is this:

Apparently a segmental retaining wall is placed right on grade. As simple as that. You just level your grade and start stacking.

That sounds great, but I don't understand how that can be that simple … if we have creeping soils and we need to go 9 feet past them and everyone (including myself) is scared to death of not drilling pier holes deep enough … how can we go from that to just tossing them right on grade and calling it a day ?

Why won't the grade move just like the creeping soils move ? Why won't the combined weight of all that wall (which will probably be canted backward 10-15 degrees ?) push the soil underneath down and out ?

What am I missing here ? When we switch to building with concrete legos, basically, why do we suddenly not care at all about the dirt underneath the wall ?

Thank you.