Doesn’t Steam also take a 30% cut?
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 think I understand why Microsoft costs are higher than others due to their a spending spree, so the break even is much higher.
You don't seem to understand the difference between the margins on a sale and the averaged margins once the studio development costs are included.
If the store takes 30% you are still left with 70% gross margins but if you have development costs, you maybe only have enough sales to break even or just barely be profitable. A 30% margin on the entire studio means the studio must produce guaranteed successes consistently.
It might also not be obvious but if Microsoft imposed 30% margins, those are a floor, whereas a fee reduces the ceiling. They are not the same even if we assume the target is met.
Game revenues follow a power law type distribution so they're hurting indies and leaving money on the table.
Valve targeted the big releases / industry players specifically because the context in which Valve added that was where all the big players were making their own store fronts: EA Original/App Ubisoft launcher, Blizzard Battle.net. And Epic Game Store, where Ubisoft (or another big player I forget) was talking about abandoning their launcher for Epic which has a lower revenue cut like ~12%. So it seemed Valve made that change to shore up keeping the big industry players releasing on Steam.