None of the other titans are challenging Amazon. They can't because Amazon's Earnings Per Share are...
-0.23. Negative 0.23.
Investors often lump AMZN with GOOG and the like but GOOG's EPS is 33.59. TGT? 4.26. Walmart is 5.07. No other company with a market cap (100+Bn) as large as AMZN is allowed to get away with negative EPS. The only one that comes close is Vodafone, with a tiny positive EPS (0.13).
It's important to note that if any other company spent until their EPS was negative, investors would flip. Amazon is playing with razor thin margins while trying to scale up a platform to end all platforms that we might someday use for everything without thinking about it. If successful, on that day/year/eon dollar bills might as well be printed with Jeff Bezos' face on them.
Amazon won't be using UPS and Fedex trucks on that day. They'll be using Amazon trucks. You'll know that era when you see it, I think.
If you're Walmart or Target its hard to justify trying to do something similar at this point, the stock could take a major dive from such a risk. They're at the "Ask-questions" phase, and the questions are always "What's the profit?" because these are publicly traded companies. Amazon has been playing it risky since the get-go. Bezos is in for a very long gamble, and that frustrates the hell out of some investors, but its lofty enough to still attract investment dollars while in the "build-first" stage. Hopefully they can pull it off for a few more years before the stock market shifts to asking questions.
So Amazon gets to play the long game that other companies are literally disallowed from playing because investors that have seen profits want more. Amazon gets to do something bold that would cause the mother of all stock dives in any other 100+Bn company. They get a free pass because Bezos is convincing and for Amazon its sort-of-always-been-this-way. Walmart/Target/Etc do not have either of those luxuries - the incredible (or believable) visionary and being a company that's still in burn (build) mode.
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All that said, an interesting question I think: What can we do to make more companies like Amazon? And is there a way to allow the older giants (such as Target) to ever be as ambitious again, without huge stock punishment?