Honestly? The US still has some of (if not) the developed world's lowest usage of contactless payment, both via cards and mobile devices. For instance, we only recently got up to 20%ish of all card payments using it--and we needed a million people (and counting) dying of a pandemic to get to that point. Why would Walmart et al enable it when (a) the vast majority of their customers likely don't care and (b) it's very possible that stuff like curbside pickup, Just Walk Out/Scan & Go type technology, etc. will eventually negate a lot of the need for tapping in the first place?
That said, that 20% figure is only an average and could be significantly higher depending on where you are. It's possible that the calculation the various holdouts made will end up changing more quickly than we think, causing at least some of them to eventually enable support.
(Personally I'm more worried about Kroger possibly disabling contactless at Safeway and Albertsons once that deal becomes final. Then again, they do have it enabled at QFC, so it's also possible they'll finally get off their butts and enable it company-wide as a result of the merger.)