Because there is no lock.

With a lock, you acquire the lock, you perform your operation, and then you release the lock.

Lock-free is typically done differently. You don’t acquire a lock, you start performing the operation “optimistically”, and you commit the result if no other thread has stomped on your data. If another thread has stomped on your data, you start over.

One of the important differences here is that it’s a race. Whoever commits first, wins. With a lock, you have to wait for whatever thread acquired the lock first. That thread may not even be running.

So, lock-free is fundamentally different because with a lock-free system, a thread that is running will always complete work (you just don’t know which thread). In a system with locks, threads that are running may be prevented from making progress by threads that are not running.

Systems with locks can also deadlock, lock-free systems cannot deadlock. Usually, there is some sort of guarantee that lock-free systems always make progress.