Generally I will bet when I get a good prediction based on my own research, for example a meeting between the two teams in football and I will see the chances of winning first before deciding to bet.
Chance of winning alone is definitely not enough and is probably the main reason why so many people lose money. What you need to be looking at is the relation of payout rate to the chance of winning (Expected Value of the bet). So what if the team A has 90% chance of winning if the odds are set at x1.05. This wouldn't be a good bet to make. And if the team B has only 10% chance of winning, but the odds are set at x15.0 - that'd be a good value bet.
Although that is definitely true that the odds of winning are relatively low on the player's side but another major reason is the habit that gamblers develop to keep playing.
Despite knowing that they are losing most of their bets they still tend to keep playing losing more. This is why the losses always overcome the profits.
This is why 90% gamblers lose money in the long term.
I would say that gamblers will lose both in the long term and short term if they have no exit plan, if they do not have any profit taking plan and if they do not know what percentage of the win needs to be reinvested in the bets and how much you should cash out according to your portfolio.
In the absence of all these things, no gambler can be a winner in a nutshell. Yes, one may get a few wins and feel he is a champion gambler but without the above-mentioned skills, sooner or later he will lose all his profits and even his invested seed money too.