It's often because the subdomain is outsourced to a third-party provider, for better or worse. https://community.godaddy.com/ appears to be outsourced to Salesforce.
I'm not an expert on Salesforce, so this is speculation, but they probably have a Let's Encrypt integration available. It's a little embarrassing, but GoDaddy probably decided it was easier to go with the default SSL setup instead of managing one of their own certificates.
Let's Encrypt isn't the only CA to offer free certificates, or an automated API, but they — and the now-standardized ACME protocol — helped popularize automated, free, large-scale certificate management among software and web services, and they remain a common choice.
Most large CAs now have some kind of ACME support (including GoDaddy, as far as I can tell), and it might be good marketing for all of your services to use your own certificates, but it's probably not always practical. It's probably not good business to limit what software and services you use by what CAs they integrate with. Any certificate will usually work, and most users don't have as sharp an eye for it as you do.
(For that matter, Salesforce probably has multiple CAs as customers! Are they all going to fight to be the default?)
If you look at https://crt.sh/?identity=godaddy.com&exclude=expired&dir=^&sort=1&group=icaid (if the website isn't too slow today), the
godaddy.com domain has over 3100 unexpired GoDaddy certificates, 74 from Let's Encrypt, and a handful referencing Salesforce and other cloud and CDN companies.
As a different hypothetical, say you're a dedicated server company. You want a corporate blog. You could host it yourself — hosting is what you do! But you have a small team of sysadmins, and none of them are WordPress experts. Your marketers are already familiar with some popular platforms from their past jobs. And modern capitalism loves outsourcing. It would be easy to pay $xx a month to a blog hosting company, set up a few DNS records and let your employees get back to work on what's important.
But there are a lot of decent server or cloud hosting companies in the world. It's not likely any of the popular blog hosts happen to use your services.
Do you run it yourself? Look for a company that will manage a blog on your hardware? Or bite the bullet, look a bit silly, and sign up with some company that will run it on hardware somewhere else?
Lots of companies choose the last option. DNS providers will outsource to services that use other DNS services. CDN companies will outsource to services that use other CDNs. Sometimes a company that provides email hosting will outsource their own corporate email to Google or Microsoft!
Life goes on (until someone has an outage and business worldwide grinds to a halt for a day).