You have a valid point about this greed, it's a huge factor in causing gamblers to be addicted as many gamblers could want more when they are winning or often believe that they will be rich in gambling which is a very dangerous mindset. However, this is not the only reason for addiction, addiction could come through the mind of placing gambling as a hobby, and whether it is positive or negative for the person (money or fun or both), it doesn't change the fact that the gambler could be addicted through it.
Dangerous indeed, but they can't help it. Sadly, it's a part of human nature, greed, and some people can't avoid or control it which is the reason why it creates problems for them at several points in life. Those who can control their emotions such as greed will tend to have more success in their lives because your greed will never let you succeed, and it will even take the success you have achieved away at times.
When you are into gambling, you need to have a lot of control over your emotions, especially greed, because your greed will not let you win anything in gambling even when you win something, when you become greedy, you want to win more with what you have already won, and in that process, you will lose whatever you currently have won.