This is very much a signaling problem. Employers are looking for attributes in candidates that are very difficult to demonstrate in an interview. So they are looking for signals that are relatively easy for candidates that have those attributes to send but relatively difficult for candidates that don't have those attributes to send. These signals can be useful even when they are far from perfect by helping the interviewer increase their odds of making a good hire. Resigning a job with nothing else lined up is generally a very poor signal because it often indicates something negative but hard to prove.
Resigning in this manner often signals prior poor performance because resignations come in very different forms. Organizations are generally very slow to actually fire white collar workers like programmers that aren't pulling their weight. Managers dislike the confrontation, the employee is generally making some contribution so letting them go makes the manager's near-term job harder, dismissals often negatively affect the morale of the remaining team members who are now anxious about whether their job is on the line, etc. Many organizations end up suggesting to the employee in increasingly direct terms that they ought to consider resigning. Often, by the time a programmer resigns, they were effectively fired. The resignation was just a convenient way for the employee to save face and for the manager to arrange an orderly transition of responsibilities. If a hiring manager looks at the resume of someone that resigned without having another job lined up, the presumption is that the candidate resigned in lieu of being fired.
Of course, it's possible to rebut that presumption. A reason that seems compelling to the interviewer, as @HLGEM suggests, is one way to rebut the presumption. A glowing reference from the previous employer is another. A call to HR is likely only to produce the dates that you were employed and your title, HR isn't going to say whether you were a top performer that left entirely voluntarily or whether you were a mediocre performer that saw the writing on the wall and resigned ahead of the axe. Your previous manager might be in a position to make that distinction but many companies have a policy that prevents previous managers from confirming more than HR would.
Resigning this way also tends to signal a lack of responsibility. If you can support yourself playing poker, I'll wager that you have a vastly larger tolerance for risk than the average person. That means that the people that are interviewing you are going to be viewing your actions through their much more risk-averse eyes. To those eyes, resigning a good job with no immediate prospect of another job on the horizon is a terribly risky move. That's particularly true when you realize that their financial cushion (which will generally be part of their calculus) is likely to be much more tenuous than yours apparently is. Most people can't afford to go without a paycheck for 5 years without seriously draining their retirement funds. And most people would be miserable if they became unemployed for even a month or two while drawing down their cash reserves even if they had them. Since your actions appear much riskier to the interviewer than they do to you, both because of your appetite for risk and because of your knowledge of your actual financial position, interviewers would tend to fear that you'd have much more tolerance for risk in your working life than they would deem reasonable.
Then, there are the issues that others pointed out. Employers generally want people that are passionate about what they do. They want people that are dedicated to the companies they work for. And they want people whose skills are up to date. Someone that walks away from a similar job to take a break for months or even years is someone that may be lacking one or all of these attributes.
This isn't to say that you actually lack any of these qualities, of course. But from the hiring manager's standpoint, a resignation with no job lined up certainly increases the probability that a candidate has some sort of latent defect. As a poker player, I'm sure you're accustomed to trying to infer an opponent's private information (their hole cards in poker, their actual work habits in the interview process) based on their public actions (when and how they bet in poker, their decisions about when and how to leave a job in the interview process). In any given interaction, you can be fooled easily enough. But over time, if you're playing the percentages, you'll make more correct decisions if you factor those signals into your estimates.
