Devs don't do it because we are a lazy bunch, which is the same reason we like using LLMs so much, because it taps into our laziness.
As evidence, I point to the massive number of mostly-LLM-generated LLM-sandboxing projects. Practically every day, I see a new 100k+ LoC project to make agentic coding safe, with shiny emoji-and-badge-encrusted readmes...
... and no issues, no forks, and usually no new commits after the first two weeks. Try to use one and you'll quickly see they're all extremely bug-ridden (one of the few constants with LLM code). And some have stood the test of time and have ten thousand issues that are mostly regressions that were once fixed but come back every other week. But they keep popping up rapidly!