There don’t have to be actual limits. People limit themselves.
Rarely are there more than 20 comments that are actually relevant to the blog. After the 3rd or 4th page, in most cases, it just becomes aimless conversation. Which can be had anywhere.
Besides, even if there are 300 comments on a blog, it won’t kill a browser to load all of them on one page. It might take a long time, sure, but those cases would be rare anyway.
I think the browsing-efficiency gains for reasonable amounts of comments is worth it. 🙁
Of note: A lot of legit blogging sites out there don’t have pagination on their comments at all. Empirical proof that they’re not a necessary feature.