I don’t the stimulus check was really the reaason sales went up as high as they did. (Though they helped a bit)
Like everyone knows about the fact that all 3 currently sell everything they ship (Nintendo more so the OG model as I never see it at retail), and like you said 5week that avg jumped up from feb 4week by roughly 25-30%. Mar10 sales/MH Rise limited edition and release and of course the end of FY that probably made Nintendo ship out lot for March which demand still hasn’t been settled
Sure, all three companies could have had huge restocks for the U.S. market in March, which could be the primary factor.We did see Sony increase stock for the PS5 in March in Japan as well, and the PS2's big restock in March 2001 helped a lot, too. And while Monster Hunter isn't a system-seller in the U.S., limited edition consoles are, so that could have contributed for the Switch.
Still, that sort of increase had to have been at least aided by the stimulus checks. We know windfalls like that help, because February is almost always the best non-holiday month of the year in the U.S. (barring some big system-seller or price cut released later in the year) due to tax returns, plus last April saw huge gains for all extant platforms. The Big Three may have dumped a bunch of stock into the U.S. market specifically to take advantage of the stimulus for all we know (though the Switch isn't exactly hurting for stock, and systems are readily available despite its current popularity, unlike the PS5 & XBS). If we still got sales data for the PS4 & XBO it could provide further clarification, but unfortunately the last time we got any numbers for those systems was in October.