The main problem is that people don't notice how much they are losing when they get some wins in between unless someone is keeping track of all their wagers and have a record of their wins and losses and then have an estimate at the end of the month or a year. But, most gamblers don't do that and only those who are responsible gamblers will keep track of their gambling activities and how much they are winning or losing in every gambling session.

So, an addicted gambler simply doesn't even know how much they are losing because they have gotten too deep into it. As you said, winning in gambling is fun and that is what addicted gamblers see only, and they don't even see that they have gambled all their money at the end of the day.