A geotechnical engineer and a waterproofing consultant tackle two different issues. Waterproofing is just that—they deal with the prevention of water from entering the space.
Geotechnical engineers provide soil evaluations that, in this case, allows the structural engineer to determine how the dirt that the house is resting on factors into his design. The two things have little to do with one another.
Waterproofing consultants are becoming quite commonplace now given the mold issues that have come to light in the past decade, and while We don’t retain one on every job, We nearly always do for habitable below-grade projects like this one. Conversely, I haven’t come across a structural engineer requiring a soils report except for larger renovations or new construction, but I typically don’t challenge them on their request unless the clients want to forego it. You could ask Local Structural Engineer whether he would be comfortable using existing soils information that might be available from the surrounding area and see what he says. It could be that Structural Engineer knows something about the soils in your area that leads him to believe that they warrant investigation prior to starting in on design, or it might be that he just always hires a geotech as a course of normal business.