I don't understand the "Amazon is eating into Facebook's and Google's business" argument, and I've seen it now over and over. Is there really a lot of overlap t

I don't understand the "Amazon is eating into Facebook's and Google's business" argument, and I've seen it now over and over. Is there really a lot of overlap there? It seems to me that the brands that advertise most heavily on Facebook and Google are the brands that don't sell much, if at all on Amazon. It seems that the overlap would be those products that are sold on Amazon and advertised via Google or Facebook, which I don't think I've seen much of. To me this seems like it would only make selling on Amazon less of a meritocracy and more of a competition for the deepest pockets. But even if it's true that there is overlap, then suppose a company cuts its Google ad budget to focus on Amazon. Then all searchers who still use Google will now go to their competitors instead. So why would they do that? If Amazon's ad business is growing it seems likely that it's coming at the expense of the sellers more than the other ad platforms.