Part of the problem is many bloggers think an image is required for every post. This is reinforced by algorithms which favor such posts, and drives bloggers to the easiest/fastest sources of free images.

The "featured image" in Wordpress is what gets shown on the index page, in shared social posts, and other contexts. And certainly, the algorithms favor such posts over pure text posts, even if the text content in the spartan post is far better.

There are other factors at play, too. Many bloggers are lazy, some are severely time constrained, a few are fearful of legal repercussions associated with copying images off the Internet, and a majority are creatively or technically incapable of making their own images to include on a personal blog post.