Amazon itself used the extremely advantageous rates of USPS for book delivery in the 90s/early 2000s to be able to grow. Combined with the lack of sales tax it could undercut basically any bookstore in sale-tax free states. Part of the reason why USPS couldn't improve itself was because of a law in 2006 which forced them to prefund retiree health benefits for decades into the future making them unable to spend those billions a year into modernisation right when it needed it the most (and private companies didn't face this pressure). Both the sale tax benefit and the USPS disadvantages are reversed now but the momentum amazon got and the slowdown USPS had can't be reversed.

The company also pushed for all sort of regulatory changes to punish competitors (e.g. minimum wage to harm Walmart) while it had labour violations which barely got fined. This isn't to say that I think Amazon didn't provide value at any point in time, with its recommendation algorithm and review system it actually abuses the added value the platform delivered in the past to benefit its own basic products, but I believe there would've always been an Amazon who would've used similar regulatory plays to get ahead. That's why it's kind of difficult for me to say how many of Bezos his billions were 'fairly' earned.

Furthermore I don't believe it's impossible for someone to truly earn a billion, the financial sector is one that has some of the best examples (e.g. Jim Simons) it's just difficult to find a 'close to purely fair' billionaire.