Large organizations’ choices influence my decisions in only one way: by telling me what users are familiar with. For instance: when building a search-results page, it might make sense to borrow the basic semantics of existing search engines (Google, Bing, etc.) so the interface is familiar.
It doesn’t make sense to blatantly violate WCAG (especially at the “A” level!) just because big companies do. The companies you cite know they won’t get sued over link underlines, even though removing them without replacement is an accessibility hazard. Every time someone jumps onto Flutter because Google said it’s accessible, I feel tempted to file yet another accessibility issue.