They're not. This isn't a problem that you can't apply normal logic to. Instead you need to look at what works and copy that - and last-minute disruption/difficulty works.
Here is an example of a suicidal thought process, from my own experience: "I don't want to die, but given the situation I'm in I can't see any other option. I don't want to do it. I just need a reason to keep going."
When you can't find that reason, you do something fatal. The reason can be anything, the smallest thing can keep you going.
Some people kill themselves for different reasons, and I think people that have never suffered from depression have that kind of suicide in mind. The guy who kills himself rather than live with the shame of blah blah blah. (Or for the insurance money for his family.)
That's not a depression-related suicide, and I suspect they're pretty rare. Someone with depression isn't thinking that rationally. If they were, they wouldn't be suicidal in the first place.