Most of the answers here mention the bone and ligaments holding the teeth in, which is correct, but it’s also worth noting that your other teeth prevent each other from falling out. More specifically, the teeth opposite on the top and bottom of your jaw. When someone has a tooth extracted, it’s common for the tooth opposite to do something called “super erupt.” Super eruption means the tooth essentially grows out of the bone because there is no longer an opposite force keeping it in place. In severe enough cases, when the bone and ligaments around a tooth are also compromised, teeth can spontaneously fall out. This isn’t necessarily rare, but for most people it isn’t a concern.