Because a blog at scale is still a complicated piece of software? You need to implement caching, DoS protection, security - something a hosted blog vendor might have experience in.
What do you mean still? It's about the easiest thing you can do... It was easy a decade ago and I reckon it's easier now. For Wordpress it can be literally a few clicks. I'm assuming they're not serving millions of users, that's when "scale" kicks in.
