At the scale Walmart operates - 25th largest GDP in the world if considered as a nation[1], supporting over 2.2 Million employees[2], in over 27 countries[3] - it's simply more cost effective to do your tech in-house.

Amazon is huge... but Walmart is truly massive... over double Amazon's size in just about every metric. They clearly have the resources to handle things on their own.

[1] https://poseidon01.ssrn.com/delivery.php?ID=0340721260780250...

[2] https://corporate.walmart.com/newsroom/company-facts#:~:text....

[3] https://corporate.walmart.com/askwalmart/in-which-countries-....