If you flip a coin million times, you will not get a million heads. It simply won’t happen. But isn’t it possible? Yes, it is possible to get heads for every million flip you make. But is it likely? Nope, not all.
Take another example: Drop a bit of black ink in the bowl of water. The ink spreads out and eventually makes the water gray. Will the bowl of gray water ever clear up an produce a small drop of ink? It’s not impossible, but very unlikely.
The water and ink is an analogy for why entropy increases. It increases because it’s most likely that it will increase, which doesn’t mean that it’s impossible to decrease. Everything that happens in the universe is just because it’s most likely to happen, but there will always a possibility of not happening that way. And “likely” always wins the “possibly” in real world.