Maybe they take the earth as an example, the life existing under very harsh conditions and deduce that it should be a good probability given enough time. But I am skeptical of this.
Try a calculator [1] and puch in your pessimistic and see what chances you actually get. It is really hard to get 10^20 without just getting 0, and we know the chances are higher than 0.
1 - http://www.bbc.com/future/story/20120821-how-many-alien-worl...
Anyway, Here is a simple description I snagged from google search [1]:
> The Drake Equation is: > N = R * fp * ne * fl * fi * fc * L > > where: > N = The number of broadcasting civilizations. > R = Average rate of formation of suitable stars (stars/year) in the Milky Way galaxy > fp = Fraction of stars that form planets > ne = Average number of habitable planets per star > fl = Fraction of habitable planets (ne) where life emerges > fi = Fraction of habitable planets with life where intelligent evolves > fc = Fraction of planets with intelligent life capable of interstellar communication > L = Years a civilization remains detectable
Now grab a pen and cocktail and make the math do whatever you want. I think that is you stick to realistic numbers you will get reasonably high values. But we might just disagree there.
[1] http://www.astrodigital.org/astronomy/drake_equation.html
> One reply to such criticisms is that even though the Drake equation currently involves speculation about unmeasured parameters, it was intended as a way to stimulate dialogue on these topics. Then the focus becomes how to proceed experimentally. Indeed, Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference.