A more practical formulation of the question is "Why do we count by tones, instead of semitones?", but its meaning could be misinterpreted; the target of the question is actually on the concept of "1 tone" as a unit: "Why wasn't the smaller "interval leap", the actual semitone, taken as the unit and therefore called "tone" (or "one tone")?
We say there're 6 tones in an octave and then that the distance of the notes of major scale (for example) jump 1-1-1/2-1-1-1-1/2 tones.
Why wasn't what is known as "half tone" called a tone since the beginning, as a tone is not any unit whatsoever? Using the actual semitone as a unit, i.e. as a tone, we would have and octave with 12 tones and then simply show that the the "leaps" go 2-2-1-2-2-2-1 tones.
Is there a practical rationale for things not being this way and being the way they are? Are they this way merely for arbitrary or historical reasons?