Just to be clear, SPIR-V has no relation to RISC-V?
It's an intermediate representation between the high level shader programming language and the GPU's native machine code. It's expected that the GPU will compile the SPIR-V to its own internal instruction set, rather than executing it natively.
RISC-V, on the other hand, is a native instruction set for CPUs.
Also it makes gamedevs slightly happier that they don't have to ship the plain text source code of their shaders with games.
It doesn't provide any real security against RE, as SPIR-V is pretty close to being tokinized/SSAed GLSL and decompilers are trivial to implement. But if it makes them happier, so be it.
SPIR-V is in SSA form; GLSL most definitely is not.
Of course, the V in Vulkan also stems from 5. Vulkan was originally going to be OpenGL 5. That plan changed, but they chose a name starting with V as a nod to that history.