There are people on HN (or probably other sites, maybe Reddit?) who will invite "strangers", but typically we want to have some idea what the person is like, as an invite creates a permanent link (on the site) between the inviter and the invitee.

If the invitee turns out to be a spammer, or posts a bunch of hate speech etc, the inviter may be held responsible for the invitee's actions. This isn't a hypothetical, it's explicitly explained in on the invite form, and I've seen cases where people were banned because they invited people who then acted spammy.