> What's missing in software engineering is the rigour and ability to reason so precisely about what's being built

That's only true if you choose for it to be. There are ways to formally prove that your software is correct but they require a large time tradeoff. For example, the software in the chip in your car has gone through as much engineering rigor as a bridge. Alternatively, look at a cheap toy produced in a shitty factory in an undeveloped country. It will have parts in it that were designed by a mechanical engineer but they choose to be less rigorous to keep cost savings low and as a result you get a toy with "bugs" in it.