The brain isn't the only thing necessary to evolve in order to start a civilization. You will also need language and the lifeform has to be extremely social, in order to pass on knowledge. You would further need something comparable to human hands to make and use tools. Another thing that comes to mind is that such a lifeform needs to be living on land. Without fire, you will never be able to make metal or have a steam engine. Electricity would also be a big problem. So I think the probability that all those things come together is what makes it really unlikely.