Because the explanations will often not be sane; when they are sane, they will focus on irrelevant details and be maddeningly padded out unless you put inordinate effort into trying to control the AI's writing style.

Ask pretty much any FOSS developer who has received AI-generated (both code and explanations) PRs on GitHub (and when you complain about these, the author will almost always use the same AI to generate responses) about their experiences. It's a huge time sink if you don't cut them off. There are plenty of projects out there now that have explicit policy documentation against such submissions and even boilerplate messages for rejecting them.