I don't understand why companies take on hosting their blog and marketing page (usually the homepage) when it isn't their core competency and a distraction for the engineering team to maintain.

It looks like this is true for Docker which seems to be hosting it themselves instead of using Wordpress.com

» nslookup blog.docker.com Server: 192.168.86.1 Address: 192.168.86.1#53

Non-authoritative answer: blog.docker.com canonical name = blog-1556129762.us-east-1.elb.amazonaws.com.