While I am sure there might be some bloggers buying comments, from my experience, many of the do not. Least the ones I've frequented. Many of them get comments the old fashioned way by commenting on other related blogs and by networking.
It's not difficult at all to get a good number of comments to your blog. Yes it is time consuming, but very possible.
Obviously, when you comment on other blogs, often the blog owner will feel the need to reciprocate, and of course most of them are also looking to get traffic from commenting and looking to network themselves.
So, age of site or Alexa rank doesn't mean anything if you put in the effort and comment yourself. You could put up a brand new blog tomorrow, doing some heavy commenting and in a day or two have a good amount of comments.
I've done it, it works. Though I am sure some may argue if it's worth the time investment. Jury is still out for me as well.
Plus there is the networking. Build up some contacts, make friends, and they could be a quick flow of comments to a new blog, future blog posts, ect.
There is one other side effect of commenting, you can build up a group of people who happily share your content on social media. It's worked for me. Now, if I was just more consistent
I can almost guarantee, in most cases, the blogs without comments are not out commenting themselves.
