The Drake equation gives the number of civilizations with whom communication is possible at any given time. This is reflected the most in the last and the first parameter. The last one is intuitive to understand in that they are the intelligent civilizations who are still around to communicate with. The first parameter is the very starting point though.
The rate of star formation is not constant, it decreases with time. At some point in time, it will eventually be zero. That is when you won’t have any new planets too and thus no habitable ones and so on. Star formation is thus the very first quantity that affects every other.