As to "how is it patentable," I think lots of other responses are probably right as to the current state of the law, but to me the more interesting question is

As to "how is it patentable," I think lots of other responses are probably right as to the current state of the law, but to me the more interesting question is the policy one: "why should it be patentable." Like, to the extent that patents are a deliberately market-distorting mechanism that's meant to incentivize invention, do they actually work as intended here? Like, does anyone seriously believe that if patents didn't protect this idea, nobody would have "invented" the idea of putting trays on a cart anyway? And assuming they would have, what's the social/public utility of having a patent system with a bar as low as ours, if it doesn't actually incentivize anything at least in the case of these most-seemingly-stupid patents?