It's due to the fact that the CEO of Walmart could Work for NOTHING and even if you distributed his entire forfeited salary to the employees it would only come out to about $15 per person PER YEAR.
So why not give the dude a raise? Even if Walmart were to decuple his salary that would only work out to about $150 per employee per year. Surely the skinflint owners of Walmart (The Billionaire children of Sam Walton) can afford to bump up their CEO's salary by a factor of ten? Heck, they could multiply his salary by a thousand and it wouldn't be a big deal.
That's why I don't get too upset about this whole "CEO to worker ratio growing" talking point (the fact that it's an essentially symbolic issue, lacks perspective, etc.) and would much prefer a more substantive analysis concerning why many of our CEOs are so woefully underpaid.