在樂譜裡，為什麼不全都用4分音符當一拍呢？

They are different counts. For example 3/4 is 3拍, using 3x 4分音符 in a bar, every bar thus has 3 units.

However, for 6/8, we use 6x 8分音符 in a bar, but a bar is not 6拍, it is call 複2拍, 3x 8分音符 represent a unit.

The 拍 concept is just for our easy use, but to play the style of the song correctly, we should bear in mind the different which the composer want to express.