You are not paying attention then. Lots of programs have all sorts of basic things that can be changed with low risk to get pretty good performance benefit, for example. You can literally just ask an LLM to look through your code and see where you were e.g. unnecessarily allocating or had poor struct ordering and fix it without changing any interfaces or functional code and have it ensure that unit tests pass and that it runs benchmarks to see that the code is faster.
You can ask it to implement the pattern you want. Like we had a bunch of gnarly intertwined logic inside of looping constructs that I could say, hey, create iterators for this, extract out the filtering logic, extract out the transformation logic, etc. I'm making the decision, but it can do the mechanical work. And again, I can tell it to benchmark my refactor to ensure that it actually cleaned up allocations instead of introducing them. It becomes easier for me to actually be more thorough in my work.
You can also use them to quickly build out prototypes for different approaches so that you can make better informed decisions at an architectural level.
You can give them a description of a bug and it will read through your code and figure out where the problem lies, even across multiple repos with complex interactions. You can of course still confirm it, but they've been superhuman at this since at least last December.
LLMs are absolutely a game changer for software development in the hands of somebody who knows what they want. For low-level things, they're an extremely solid junior (they don't ever really make mistakes, they just write tasteless crap). For high-level things, they're an extremely solid discussion partner with wide knowledge and great reasoning skills.