In fact God has made our ears to function properly our ears can't hear the unnecessary voices and noise. For example, if you are in a noisy environment you can not listen at all what is being said, you would just hear the noise. And it is a blessing indeed for us.
Our ears can only hear the voice which is in between the frequency of 20Hz to 20000Hz. If a voice or sound is higher or lower than this frequency then in this case we can't hear that sound.
And this is the case with the footsteps of the ants. They produce the sound which is very low in frequency. And due to this reason we can not hear the sound made by the ants. I think my answer is very logical and you can understand it. You can also not hear a voice if its frequency is higher than the limits I've mentioned above. That's the reason dogs can hear the sounds which we can't and you can observe many other animals doing this.