A likely reason is mechanical simplicity and reliability in the iris mechanism. The aperture diaphragm is a very precise moving part, and adding more blades increases complexity, friction, and the precision needed for smooth operation. Using fewer blades can reduce those demands and make the mechanism easier to manufacture and operate consistently.

So even on expensive lenses, blade count is not only about cost or “better bokeh.” Lens designers balance many factors, including mechanical reliability, smooth actuation, durability, and overall optical design. A polygonal out-of-focus highlight shape can be a visible side effect of that design choice.

Also, whether this is a “flaw” is subjective: some shooters dislike obvious polygonal bokeh, while others accept it as part of the lens’s character.