I accept only hobby as an answer lol
everything else is, arguably, wrong! Ask any software engineer! Like me :p
I am lazy to go through all of them and explain why...
But hey I respect his choice
The argument about it being hard to split credit and payment applies as well.
Working with code is very easy. It just sits there quietly and is only wrong if you mess up. Working with people can be easy or it can be hard. You may have had a happy experience with it, but as a coder, I can very much say that it's not always smooth sailing working with other people. Toady does everything just fine right now, there's no need to spoil that.
Once you bring in more people, you have to also start training them in your artistic vision. He can just say, "Hey, I want you to implement proper farming in adventure mode." Even if he says it in fine detail, other coders could easily get the wrong idea. It's different than telling coders to optimize code, or implement some kind of secure payment system. Any kind of artistic work requires a lot of communication and coordination, and there's more work involved in bossing people around than actually doing the work yourself.
He could of course pull it off if he were to do some full time supervision. But then it becomes the collective creative work of other people. If I had my own megaproject, I'd probably go this path, and I think Sid Meier uses a similar technique. That's only because my strength is that I deal with people better than things. But it's not really applicable to Toady's style.
So, no, not just "hobby", but because it's easier this way. It's funny that software engineers keep bringing this topic up, when it's more comparable to movie directing, and I suppose professional game design supervisors.