Think about it; being on their back is a vulnerable position, and they are prey animals - tame or not, it is incredibly common for the majority of hedgehogs to tolerate being in this position for more than a minute. Many of them also do not like having their stomach rubbed, or even being pet on their face/visor.
With social media, you get false expectations. Yes, there are some hogs who genuinely do not seem to care about being on their back, and having belly massages - but they are a drop in the ocean, and it is not common behaviour for even a very tame hedgehog.
Also, kind of unrelated but also not; am I the only one who thinks it looks like the majority of these hedgehogs are being touched/inappropriately stimulated? I've always found it a little unnerving that it seems the majority of hogs who like belly rubs are males - and i've wondered forever if there's a connection somewhere to how they're being petted in their genital region.