transpose and adjoint are linear algebra operations. If you want to treat your array as a container, use the container operations like reshape, vec or permutedims.
As far as the why goes, the answer is that LinearAlgebra wants to support matrices of matrices and not all LinearAlgebra objects are themselves ::AbstractArrays — so defaulting to a no-op is subtly error prone and defining a bunch of no-op transposes one-by-one is unsatisfactory. I’m not the biggest fan of this myself, but it is what it is.