Unity’s physics mimics real life. There is nothing in real life that would make a ball that is dropped from a height onto a uniform surface to bounce to the left or right. It will bounce straight back up.
In order to make the ball bounce to the left or right the way you want to, you will have to write additional code to handle that behaviour. Start first by simply making it bounce in one particular direction all the time. Once you get that working, you can think about getting it to bounce in different directions depending on where the ball lands on the paddle.