Because it is a well known site.It is that simple.Most of the times we join a site to earn but it turns out to be a fraud.

How to earn from Google.com?

Make a site on any topic which you are good at.
I have made one site http://www.oocities.org/onlinemoneytransfer
I have lots of knowledge regarding online money transfer and thus have made a site based on my knowledge.You can do the same.

How to make a site(your logical next question)?

It is very easy.You do not need to know any programming language .Just join Yahoo Geocities http://geocities.yahoo.com like how I did and they will guide you step by step on how to make a site.
I did take effort to learn basic HTML and modified my site

After making a site now what?

Now you have to join Google adsense.It is free to join.

What is Google adsense?

It allows you to post google ads on your site and if someone clicks on the ads ,you earn.

Is that all?

Yes.That is what I did. Check out my site http://www.oocities.org/onlinemoneytransfer It is a simple but effective site with google ads placed.

How will people know of my site?

Well, if your site is good you will get hits.There are many ways to promote your site which you can find on the net.

THROUGH THIS SITE I HAVE JUST PLANTED THE SEED IN YOU AS TO HOW TO EARN WORLDWIDE FROM GOOGLE.COM.NOW IT IS UPTO YOU TO FULLY UNDERSTAND THE CONCEPT AND MAXIMISE YOUR EARNINGS.

GOOD LUCK