Might be because lots of us are not using a CMS. For a simple blog page you don't even need a backend - just generate static sites with jekyll.
Having a 3rd party solution for comments is thus necessary (if you want to have comments).
Having a 3rd party solution for comments is thus necessary (if you want to have comments).
Also by the time you outsource a comment system due to CPU your site probably is big enough for a "real" CMS.. or atleast better hardware, it's not like a good vserver is expensive nowadays.
For example, I have a friend who runs a small business selling music samples. He uses WordPress because it is a simple solution that can do a lot for him without a super amount of technical savvy.
I think WordPress is disgusting, but the point is, there is a market for these products. If you feel strongly against this trend, you should consider developing one that drives your vision that has similar practical value.