Well, Walmart is quite well resourced, has a lot of talented folks, the facilities (for engineers) are enticing, they offer the opportunity to work on stuff with huge scale, and the pay is relatively comparable.
What more do you want?
Apple, Google, Facebook, Airbnb, Uber, Lyft, etc. etc. offer all of these things as well.
The only way for Walmart to win the talent game would be by offering absurdly above-market pay packages or absurdly above-average work-life balance policies. Which I'm quite certain, given their corporate culture based around beating down costs and maximizing efficiency, will not sit well with the lifer-execs back at home base.
Fun fact: If you account t for cost of living, Wal-Mart does pay more than many others.
There's also many other perks from working at home base. Massive fitness center (Who else can can claim they have 3 pools), cheap Sam's Club memberships. Add in that you get to try all the cool things like most others do (Savings Catcher, Scan n' Go, Grocery Pickup) and you get a substantial boost to the value.
Also, supposedly the new museum is amazing, for what it's worth.
BTW, we're hiring a mobile release engineer for our small Portland office if you're interested.
Good luck getting a talented engineer without either an Ivy League pedigree or a history of employment at AppAmaGooBookSoft hired at any of the trendy startups you've listed.
SV is a bubble, and there are good people outside of it.
Walmart is a different experience in the places where there isn't social stigma attached to it because normal people don't have any problem going there, but also because there isn't a political apparatus working to deny their attempts to improve their buildings, expand to newer areas, etc.