Because it makes economic sense to apply extremely rigorous standards when writing software for a space probe, but not for a web app for expense reclaims?

Or maybe because we are, literally, talking about rocket scientists?

Perks of direct control over your systems.

When you inherit some random security update to an obscure library and it causes things to break, you were never expected to understand that edge-case.

However, if you spin up your own safety critical embedded system, you should be able to follow the logic completely.

Different costs. One is in the billions of dollars, one is in the hundreds of thousands of dollars.

True, yet I feel the expectation regarding software has shifted from expecting much better quality even 10-15 years ago to things we have now, where nobody is surprised about web apps crashing your browser etc.