Why Height Is Measured in Feet and Inches Combined, Not Just Inches
Height is conventionally expressed in feet and inches together (rather than as a single unit like total inches) largely for readability and familiarity — "5 foot 9" is more immediately intuitive to most people accustomed to that format than "69 inches," even though both describe the exact same height, and this convention is specifically why height conversion tends to require the two-step process covered in the conversion guide.
This is purely a convention, not a mathematical necessity, but understanding why it persists explains why height conversion has its own specific extra step compared to simpler single-unit length conversions.
Why the combined format persists
"5 foot 9" is widely understood and immediately meaningful to anyone accustomed to the imperial system, in a way that "69 inches" — while mathematically identical — generally isn't, since people don't typically think of height in a single large number of inches; the combined format matches how the imperial system is actually used and understood conversationally.
How this compares to other length measurements
Other lengths (the length of a room, a distance traveled) are more commonly expressed in a single unit (feet, or feet and a decimal) rather than a combined format — height's specific convention of combining feet and inches together is somewhat unique to how people conventionally describe a person's height.
What this means for conversion
Because height uses this combined format by convention, converting it accurately requires the extra step of combining feet and inches into a single unit first (as covered in the conversion guide), unlike converting a length already expressed in a single unit, which converts in one direct step.