Well they're interested in all of those.

But conveniently, there's a legal document that establishes a relationship with your spouse, so that's really easy to find.

And so it's kind of an extra level of stupid for getting caught...

It doesn't per se. The Insider Trading rules at my company speak about anyone living in your household as well as any close family members. Guess that's because there is reasonable assumption you'd be able to tell those people about internal knowledge.

Lots of people who do insider trading think "if I don't do it then I won't get caught. I'll just have my [wife, dad, brother, cousin, mother-in-law] do the trade and the SEC will never figure it out".

Because (excluding some explicit agreements where they're legal), you share finances with your wife, so if you get money, so does your wife. Possibly the same with your girlfriend. A friend or random dude on reddit would need a proven connection.

Probably doesn't. Probably just makes it easier to automatically flag.