How the fuck nobody notices some randoms coming to steal snacks in the first place ?

There's a huge difference between a company with its own building, and a company that shares a building in some way with other companies.

Many I've seen have it setup so that if you get past the security guard at the lobby, you effectively had full reign of the entire building, including many companies that wouldn't lock the doors or common areas.

I worked somewhere with a few hundred employees across 3 floors. If someone wearing business casual walked onto our floor I would have no idea if they worked for us or not.

~400 person company spread across a few floors, but only one kitchen. It wasn't weird for people you didn't recognize to come off the elevator and get snacks to take back to their floor.

I work at a company of ~200 people and I already don't recognize everyone. Seeing an unknown face, I just assume they are from some distant team that I never had to interact with, say hi and move on.

We have nearly a 1000 people in my building. I don't track every rando that walks by, nor reasonably could I.