I think many players withdraw up to 85-90% of their winnings because they don't want to keep the money there. They use the rest for necessary bets, such as football matches.
This goes back to human nature, which is easily forgetful when they're enjoying themselves. They shouldn't complain when they're having a bad day.
Gamblers do this are responsible gamblers and they practice gambling resonsibly so they are not addictive gamblers. I don't know from which sources you got this figure or it is only your guess and even I don't have any source to prove this, I believe that most gamblers are addictive more or less and only minority of gamblers can bet responsibly and are able to manage their bet action, bet capital and finance well enough.
I believe that the figure would be much lower than withdraw 85% to 90% of their winnings as irresponsible and addictive gamblers would reserve all or most of their winnings to continue betting.