Why?

-Blogger is owned by Google
-AdSense is owned by Google
-Search engine optimizers say that Google is where most of the searches and website traffic comes from.
-There has been theories that say that Google favors Blogger sites; This could mean possible higher rankings that having your site not on Blogger.

Those are just a few reasons.

However, if you’re planning to build AdSense sites the right way – by building content-rich sites, that takes a lot of time (and money if you’re outsourcing it).

I would suggest to pay for some affordable domain and hosting and host your site on it. Prices on both of these have dropped considerably in the recent years.

If that’s not an option, build a free site until you earn enough money to pay for them and then switch over. Be warned though. Changing the site over is a big hassle and can result in loss of web traffic.

Google Adsense are just javascripts so all hosts will support them, unless they block because of terms and conditions.

Google Page Creator allows javascript now so using Adsense with them is easier.