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Back in the days only the rate of star formation $R$ was know, telling how many stars are born every year in our Galaxy. This number is about 7 and might seem quite small, but the Galaxy is billion of years old, so plenty of stars have born (and died) in the meanwhile. The other terms in the Drake equation were not known in 1961. Actually till the early '90s we didn't even know if planets were a unique feature of the Solar system. The problem is that if life was impossible around stars noone would know it. Having one data point, the Earth, doesn't give any statistical information: the solar system could well be an extremely unlikely, or even unique, place in the Universe. As soon as you ask the question "Are we alone?" it means you are alive and conscious, i.e. you are in a sweet-spot, and you can not make any claim about how common that might be. Beside the fact that one such place exists. The cool thing is that, statistically speaking, finding just another place where life can be supported around another star changse changes  everything. And this is where it gets exciting.