Valve isn’t trying to make a bunch of games with AND pay for/off all the studios they bought, all while trying to advertise and compete with Sony and Nintendo in hardware.
Steam just sits back and takes money, for the most part. The hardware/software/advertising spend is probably a drop in the bucket compared to MS.
I initially misunderstood OP. However the more I think about it, the more it seems similar though. If Steam is charging game developers a 30-20% cut, that must mean those game developers margins are higher than 30-20% or there will be no point selling on Steam. It seems that Xbox would be expecting their Studios to operate at the same efficiency as others in the market then, no?