Bitcoin is very secure, but exchanges have been hacked and coins have been stolen that is why we must always do our best to protect our coins since there are always hidden dangers that we could not be able to predict.
That is correct, we did not know when those exchanges will be hack and get offline. Too unfortunate,but it really happens.We need bitcoins since it becomes like a de facto currency online and also possible in the future.But, we must also be very vigilant and cautious in storing our bitcoin on online wallets.