Another way to look at it is that the servers storing this data are constantly being fed new data. This means that the oldest data expires at a certain time, and falls off the server permanently. Your ISP may have only the most rudimentary Usenet access, and they may only retain new files for a couple of days - maybe less. If you aren’t there to get them when they come in, you’ve missed your window.
My brother also uses Usenet. Awhile back, I was helping him set up his new version of Agent reader. He had to download a list of the newsgroups his ISP carries. The total was somewhere less than 15,000 groups. The Usenet files on that server last a few hours. The premium server, for which I pay a small fee, carries close to 110,000 groups. The retention has just been increased to 100 days for binaries, and to forever and ever amen for text messages. So you can’t begin to tell what’s out there unless you have a decent NNTP server to feed the groups to you.
Just remember that Usenet doesn’t come with a search engine. You have to browse through it to find the groups you’re looking for and subscribe to the ones that interest you, then download all the headers from each one, to see if anyone has posted what you want to find. This is where I must mention that if you are on dialup, FORGET IT! Downloading the headers for one group could take all day, or longer. There’s obviously a learning curve, but I managed to overcome it, and once upon a time, I’d never heard of Usenet before, either.