@wuerling's answer is right.
There is another reason.
Consider similar situations in English:
shit --> shoot --> shucks --> sugar,
hell --> heck,
god --> gosh.
Minced oath substitutes or euphemistic expressions formed by misspelling, mispronouncing, or replacing a part of a profane, blasphemous, or taboo term reduce the original term's objectionable characteristics.
Extension of this phenomenon in Chinese:
肏 --> 操 --> 草 --> 艹 --> 擦（cā） --> 嚓.
So you can also see phrases like: 我擦，我擦嘞 online to express exclamation or amazement or shock or sometimes outrage.
This is also an attenuation of the pronunciation, less plosive.
Note that 擦 --> 嚓，changing 扌(hand 手) to 口（mouth）even attenuates the meaning from making a motion to just producing a sound.
For the purpose of academic discussion, 日, 干 (do) also mean f*** in this case.
Examples: 我日你。我干。