Because when you bet in lottery, there is no addiction and a lot of stress to be involved. The chances may be slim to be able to hit the lucky numbers but at least you are not going to lose or sell everything that you have unlike what happens when you have gambling problems in traditional casinos. I also believe that if a huge money is meant to be yours then you will receive it.
Oh I believe there are a lot of people who are getting addicted in lottery, just in a different way than the casino addiction.
For example in a casino, you might lose a few games and then decide to keep playing until you lose everything or get back what you lost... that's an addiction.
In lottery you keep playing for years, losing a few money constantly.
The results might be the same in the end... though your chances of wining in casino can be higher if you play a game which includes skills.