Padding is not used as it is just wasted computation. The network is not learning anything from those pixels.
Also, when we have to resize our images. There are 3 different approaches

Squish them:- In real life we do not see squish images, so working with these does not make much sense. As images are distorted in various ways.
Use padding:- usually try to avoid it, as you can do much more by using a small image size
Random Crop:- This is the preferred appraoch. This allows your network to learn and generalize better.

If you have a dog image. With random cropping, you can use different regions of an image to train your neural network, which forces the network to generalize. Also, as you said it is a good data augmentation technique.