The question posed by the OP comes up frequently, in various forms, on the DIS Boards and I always get a kick out of it. This is not a slam to the OP. You can look at any lifestyle indicator and ask the same question. For example, how can my neighbor afford a Hummer, or a $700,000 home? No matter the issue, it seems to come down to this:
1. The person makes a lot of money, or inherited a lot of money, and can do what they want with it.
2. The person can't afford it, charges it, and is in debt up to their eyeballs.
3. The person values it (vacations, big homes, cars, whatever), so works their budget, and maybe cuts back in other areas, to make it happen.
Everyone here at the DIS is so diplomatic and polite, but I would really love it if someone would respond to one of these posts "I make a crapload of money, so I can go to WDW anytime I want", which likely is the case with some. Anyway, for me, it's #3. I value vacations, so I build them into my budget.