The simple answer is: because every single dollar/pound/yen you're not paying yourself, you could be using to grow your business.

Imagine you're semi-profitable, you've got enough to pay wages for a few months while you finish off your next iteration/product and bring in new cash. Then your project gets delayed, and all of a sudden you're running out of cash to pay the actual programmers. Do you try to make them take a cut and risk them jumping ship, or do you cut your own salary to close the cash gap?

These sorts of things happen with (from what little I know) quite alarming regularity. The founders are almost certainly taking out less cash than everyone else up until the equity starts being traded.

The same sorts of decisions can occur even if you're not desperately squeezed for cash. Do you bump your personal wage up, or do you use it to hire that kickass designer you just found who can really polish up your stuff and give you a better chance at success?

The whole founder-as-martyr approach explains why they feel justified in taking out early cash, and why there's a general feeling that '1st employee' shouldn't get any early cash - they've already gotten their fair share through their wages up til now. It gets a lot more complicated when your employee is taking a pay-cut to work for you on the hopes you'll succeed, but then it's up to you and them to negotiate some sort of fair compensation scheme when you do get cash in.