I think people say this because there was one specific 6TB model that does really poorly in BackBlaze reports, combined with a generally poor understanding of statistics ("I bought a Seagate and it failed but I've never had a WD fail").
I will also point out that BackBlaze themselves consistently say that Seagate and WD are pretty much the same (apart from the one model), in those exact same reports
from context, it's probably the "single dude in nebraska holding up the entire internet" one
e: holy shit I got the state right