And they do so, in the form of Wordpress.com. If what you want is a blog about your dog, you can have it for free with Wordpress software that you never have to manage the code for or pay for. I have a number of friends who use it and they couldn't be happier. In fact, there are plenty of other SaaS vendors out there who are equally happy to provide you with a place to write about your dog.
On the other hand, if you're running your own web application, you're responsible for it, just the same way you're responsible for your computer. As I said, I don't blame Microsoft if I download random software and that software does malicious things. Similarly, I don't blame Wordpress or Automattic if I download a random theme and it contains malicious code.
I can not follow what logic keeps you from blaming Microsoft for the security lapses in the Microsoft operating system. Of course they have cleaned up their act in recent years, but they had a solid 25 year run (both DOS and Windows) where they were absolutely notorious for poor security. They faced sharp criticism for the many flaws in their OS, and for the low priority that they gave to implementing something better. Do you want me to google some of the many articles on this theme? Here is one from a few thousand results that I could pick from:
My criticism of the WordPress team follows the same logic as the criticism of Microsoft. Since Microsoft came up with an OS that had a lot of security flaws, it was reasonable for people to criticize Microsoft for those flaws. And since the WordPress team is producing software that also has flaws, it is reasonable to criticize the WordPress team for the flaws in the software they produce.
On the other hand, if you're running your own web application, you're responsible for it, just the same way you're responsible for your computer. As I said, I don't blame Microsoft if I download random software and that software does malicious things. Similarly, I don't blame Wordpress or Automattic if I download a random theme and it contains malicious code.