score:10
Your question is full of unwarranted assumptions that deserve being pointed out:
As Kris explained (+1), the latter is the most likely explanation. In fact, flying without showing any ID is reasonably common, including from some of the countries you mentioned in your comments. Whenever there is a check, in my experience (and with one notorious exception: Ryanair), it's only an ID check and not a check of the person's immigration status (i.e. third-country citizens who require a visa are not asked to show their residence card, only the ID page of their passport).
Upvote:10
ID checks at the gate are so called "conformity checks". They check whether the person presenting herself at the gate is indeed the same person as the person that is on the passenger manifest.
Whether these checks are done depends on airline policy, and national requirements. Some airlines do this for all flights, because they want to prevent resale of tickets. Low cost airlines typically always require ID. Some countries require all airlines to do such conformity checks.
But it is by no means universal. I fly SWISS and LH a lot, usually from Zürich, and never have to show a passport or ID at the gate. When flying Brussels Airlines however I almost always have to show a Passport or ID.