I would say it's because history teaches us that goodwill only gets you to a certain point.

When you're acting on good faith, asking for a formal contract should be seen as a totally normal request. If the other party is also acting on good faith.

This is basically a win/win situation. If they complain about such a simple request, they might be hiding something. And if they do agree to simply formalize the agreement, you're protected.

I've been burned by "good faith" before, and it changes you. By pointing out "rookie mistakes" maybe someone will read this and start taking these things a little bit more serious.

Of course on an ideal world, we'd be protected and shouldn't have to worry about it. But that's utopia for me