That feature was not always available. As a result, many Blogger users added a third party commenting system. I use a third party add on commenter myself.
I wrote yesterday about the importance of comments, but failed to discuss the Blogger commenter.
While many of my readers don't use Blogger, as their blog's host, many others do.
For their benefit, check out my friend The Blog Bloke's excellent discussion, on the many ways to use Blogger comments.
It's well worth reading, even if you don't use Blogger, because you will easily encounter many Blogger hosted blogs in your surfing.
Happy commenting.