Why does a lucky dip on the Lotto not include a bonus ball number?

That's not how it works fella - the bonus ball is drawn at lotto HQ to work in conjunction with the 6 numbers on your ticket be it lucky dip or 6 of your choice. If you ain't got the first 6 numbers, hence the jackpot, you have a chance to match any 5 numbers from your 6 plus the bonus ball to win a significant amount.