Such systems just render your conveniently created post content into a static HTML website than can be published on various free systems (github pages etc) or $2/month VPS. Or an Amazon S3 bucket.
Why?
1. It is not anyore significantly harder or more inconvenient than using a 'platform' like wordpress.com
2. A static site is 10000000% more resistant to idiotic exploits (which unless you are some kind of PROFESSIONAL blogger, keeping up with is more trouble than its worth on any dynamic 'blogging platform' type of system)
3. A static blog system can serve your blogs with any web server in 2012, and will be able to in 2112, whereas none of the free or even for-money 'blogging platforms' is likely to exist in 2112. Which in terms of your life (unless we are lucky and get the gerontological life-extending technology soon) is essentially 'forever.
4. Why on earth would you cede control of your personal output to walmart.com or google+ or similar?
[2]: http://jekyllrb.com/
EDIT: meant to mention somewhere in there that comments, one of the raisons d'être for these dynamic blogging systems, can now be superbly handled by off-site add ons like disqus.