At first I thought it might be so that you could still access their website for support if you had an issue with their CA.
However it seems that https://community.letsencrypt.org/ which would seem a good place to get help is actually using one of their certificates so maybe it's just historical. The certificate dates back to 2015 and expires in next February, maybe they'll replace it by one of their own then.
We set that up before we were an operational CA and it has just never been a high enough priority to replace it. The team that would do it has a lot of other things to do. We'll get around to it at some point.
However it seems that https://community.letsencrypt.org/ which would seem a good place to get help is actually using one of their certificates so maybe it's just historical. The certificate dates back to 2015 and expires in next February, maybe they'll replace it by one of their own then.