Because he's not taking the godawful software with him. That's staying.
It's also a problem for me because he's done his damage. I want to make high-quality software, on time, that does what the customers need. I want this out of both a sense of personal pride, and also because I need the company to keep making sales so they can keep giving me money.
I don't run the company, and even if I ignored everything I was told to do, and didn't get fired, and instead followed sensible, professional processes, I don't write the entire software by myself.
I am bemused that someone of your experience has trouble grasping this. I thought perhaps you were some kid straight out of school who thinks the whole world works like the marketing material for agile, but it turns out you've got a decade of experience. You must, surely, have seen (or at least heard of) companies that don't look like the marketing material for agile?
Sure. Mostly when I see it, it's people who don't know any better. With a fair number who do know better but do it anyhow. The former group I understand. But the latter is a mystery to me.
It's also a problem for me because he's done his damage. I want to make high-quality software, on time, that does what the customers need. I want this out of both a sense of personal pride, and also because I need the company to keep making sales so they can keep giving me money.