I just want to demonstrate how an argument of "great numbers" is a fallacy when it comes to prove/disprove something. Now, I'm not going to argue whether aliens exists or not, I just want to show that "there are X items, therefore one of them must be Y" is a false argument, without knowing the actual probability of Y.

Consider a school where every kid was given an Euromillions lottery ticket. The next day, a 6-year old says to his parents "I can't wait for them to annouce which kid in our school won the lottery!" "Well, probably nobody" his parents say. "But how can that be??" says the kid "The school if full of classes all the way from grade 1 to A-levels. And there are tens of students in each class. Surely someone must have won."

By simply repeating there are billions/zillions/kajillions number of stars in the galaxy, if we do not know the actual probability of "winning" then we are just like this kid.

That's the problem with large numbers; they become difficult to image, and we start thinking "oh it's bajillion". "It's infinitely large". But ultimately they are just finite numbers and one "bajillion" can be to another "bajillion" like a grain of sand compared to the sun.

So, I'm going to give an example using the numbers mentioned here. 70,000,000,000,000,000,000,000 seems like a huge number.
Now (and this is just an example), lets assume that for life to exist on a planet there are 20 factors (temperature, humidity... etc). For each factor there is a 5% chance that it would be "right" for life to start, and if all 20 factors are "right" then that planet will have life. Then the probability of a planet being "right" for life is 1 in 10^26, which is more that 10 thousand times larger than our 70,000.... etc
If we decrease the %age to 2% per factor, the number becomes 10^34, which is a thousand billion times larger than the number of stars. A THOUSAND BILLION!!!!

Or consider each planet having a 10x10 chessboard with each square being randomly black or white. And someone says "well! With soooooo many stars and planets, surely one must have a chessboard that's exactly like the one on earth!"
Do the maths...

Again, this is not saying that aliens don't exists. I'm just pointing out the fallacy of throwing large numbers. Without the probability of life forming, these numbers are meaningless. And can life form easily? There are indications that yes (looking at cellular automatons for example strongly indicates so). But there are also indications to the contrary. The truth is, that given our limited resources (we only have "access" to the few planets in our solar system) it's difficult to conduct reliable simulations for the time being.