One reason they report in MW instead of MWh, is that grid operators primarily spec systems by their continuous MW, and only secondarily mention their cumulative MWh. So that's the way they talk about them, and reporters just mimic that. Both are important, but the top-line number is MW, for sure.

One reason for this is that for a lot of grid-support functions, instantaneous or short-term power is vastly more important than long-term energy capacity. The other reason is that traditional generation is only sized by MW; you just keep pouring fuel into it forever, so the notion of MWh doesn't even enter the picture. They're not even thinking in those terms.

You and I, as battery natives, think about it the other way around. But we're not grid operators.

Yup! Some breaker trips, and a 500MW plant goes offline. How much battery storage do you need to compensate? 500MW. The fact that it stores 500MWh and therefore only lasts an hour comes afterwards, that is plenty of time to ask all the other fossil power plants to scale up a bit.

It's probably also a view that is shifting as batteries move from primarily providing short-term grid stability (where power available is probably the more important metric) to longer term storage.