Because most music is written in an Octave comprising of 2 bars of music.
An ascending bar and a descending bar .
And, by the way, we do NOT always count in 8s-- to the contrary, thats the old stage chor. way of counting down to the next verse or chorus ( 16 and 32 bar segments )
salsa teachers (?) tend to count in 8s. because most are not ballroom trained ,and have little or no formal dance training .
Lastly-- many figures are structered over 8 notes .