Because they need it to work, so that everything built on it works too.

Building developers sell you the apartment, not the elevator room, the electrical room, mechanical room, etc. They will make all sorts of controversial decisions with the apartments; odd layouts, ugly flooring, weird pricing, tacky finishes, etc. The "core product" is the money-maker, that's where the egos clash, priorities change, and where they try to charge as much as possible while they cut costs as much they can.

No one is buying the electrical room though. It just has to work. Yes, you'll make it as cheaply as possible; no flooring, no paint on the walls, no interior designer meetings to argue what's the right tone beige for the walls. But it'll do what it needs to do. It'll keep the lights on. Otherwise you can't sell any of the apartments.

Same thing with Facebook; there's active incentive to introduce all sorts of dark patterns over their app, to ignore certain bugs, to unnecessarily change things, etc. But none of those incentives are present with React. The incentive is to keep React reliable and performant, and to keep the team lean. I'm sure it's similar with Bun in Anthropic.

And to be clear, Anthropic definitely spends most of it's engineering effort making sure their core product "functions properly". This "functions properly" is just different for us as clients vs them as a corporation. There is high overlap, since they need to keep us clients happy. But a well-functioning product at a company is one that leads to money. I'm sure very capable engineers pushing the okrs they care about.