One factor that's seemingly always overlooked by people inventing formulae for estimating "intelligent life" is the question of when. For example, we've just developed moderately sophisticated technology and life on Earth has existed for around 4 billion years. There was absolutely no inevitability about our existence. No other species has ever developed complex tools. Perhaps no species ever would. So even with the most wildly optimistic notions about the eventuality of such life appearing elsewhere, it would need to overlap with us. And that's practically zero probability. Hence we are likely to all intents and purposes absolutely alone in our neighborhood.