The Walton family hold a bit more than 50% of Walmart shares. This is by design. As they control the board, they usually make sure that Walmart buys back enough share to keep them in control. Their income are mostly coming from dividend, which amounts to a few billion a year :) It is possible to argue that this basically prevents Walmart from taking risks in more innovative business lines.
Jeff Bezos holds less than 20% of Amazon now, but he basically "owns" Amazon for all practical purposes. I'd also say that at least 20% of Amazon valuation is based on Bezos being the CEO/President/Chairman of the Board.
That's an odd sentiment. Walmart is more distributed just because Sam Walton was dead. As the time he was alive, it's even more concentrated than Amazon has every been.
Why does it matter? This is probably true for most VC backed companies. Founders have loins share of equity. Why is it unfortunate? This is how VC backed economics work or more generally how capitalism works. Risk takers reap most rewards.
I think it's unfortunate if it is.