The Smart Shapes are a new(ish) feature. These are developed using JavaScript. The older shapes are hard-coded and have been the core design of the Xara codebase.
Xara is driving towards web delivery so the Smart Shape paradigm will probably become more prominent; all this will take time.

Smart Shapes are code that do not existing in earlier versions.

Xara should never have presented a bog-standard rectangle as a Smart Shape until it was in a position to meet your very valid expectation.

Acorn