Earlier quoted context omitted.
Really simply? Because it's worth it. Nobody's "bowing to" Steam, they could sell off their own website for free, if they wanted. But Steam offers enough of a marketplace, enough tooling, and enough promotion that it's worth it to sell there. It's what I want for the Apple App store, too. Don't charge any less, just make it worth it.
My wording was probably dramatic there, it's just I don't think a Microsoft subsidiary would lightly take the decision to forgo 20-30% (TIL it's tiered) of their revenue to a third party when they have capabilities to handle such services internally. Building platforms is kind of Microsoft's business, so to me migrating Bethesda to Steam is their acknowledgment that a competing platform has won that space. Seemingly…
That said, I did forget until now that GamePass exists - Microsoft isn't really challenging Steam anymore, they're doing kind of an end-run around them with their games subscription service, and it frankly kinda rocks. But for those who still want to outright purchase games, they still sell on Steam, because, again, Valve have made it worthwhile to pay the 30% to at least get the userbase and marketing bump.