Google is a database & search engine. It searches it's own database which gets updated ona batch basis every so many hours. That takes a bit of time.
Newssevers send the posts to other newsservers and those send them to their subscribers as well as taking new posts from their subscribers and sending them to other newsservers. Not all newsservers carry all newsgroups by any means. And they also have a "latency" period after which old posts expire and are purged from their storage.
Newservers are thus faster but don't maintain much in the way of long term archives. Google is mostly for archives & searches but there are also many groups Google does not carry. Being aboe to post from Google is sort of an add-on to the original purpose. It's not a newsserver.
Now I have a question: What other archive Sites are there besides Google?
HTH