why on Kubernetes for the short name (k8s) they replace ubernete by 8?

The same reason Accessibility is written as A11y. It's easier.

It also makes a better hashtag in posts, though this benefits A11y more as it's a generic word.

"ubernete" is 8 characters long, that's why. I always thought that it's pronounced something like "keights" which sounds kind of like Kubernetes, but it's not really like that.

Same as i18n as in internationalization