Having human-readable text as the lowest common denominator is a laudable goal. Shell scripting would however probably be improved if most tools offered alternative typed streams, or something similar. I am not convinced Powershell's approach is the best, but their approach is at least interesting.
The original decision was about not proliferating specialized or proprietary binary formats, which was more of a norm back in the ‘70s than today. The goal was to make small single-purpose tools that communicated through a common interface (files) in a standard format (plain text). Unix succeeded and continues to succeed at that.
Nothing about those design decisions precludes tools using binary formats under Unix — image processing, for example. It just precludes using standard text-oriented tools on those formats.
You’re right that standard Unix tools don’t have a concept of types in streams, but that decision got made deliberately. Types and formats got left as output details.
Analogously my Macbook Air doesn’t have a fan, by design, not by accidental omission.