The companies don't monitor torrent sites, they monitor the torrent trackers (which is what connects people together to share the files you're downloading). Basically they connect to the torrent trackers, search for copyrighted content, and then collect the IP addresses who are uploading that content. Then they attempt to download a piece of the copyrighted content to compare it to their own piece to confirm it is indeed copyrighted content. If it is, they note the IP address, the content that's being downloaded, and the time it's downloaded. Then they identify the ISP from the IP, and send the ISP the notification that you're downloading illegal content.
There are a few ways to sort of block them from doing that, but they're not 100% foolproof.
One way is to block all the known IP addresses the companies use to track the content so they aren't able to download anything from you. This doesn't work 100% because IPs can always change.
Another is to use private torrent trackers, as you have to be registered to a site with it's own tracker that generally allows users to only sign up via an invite from another member. Since no one is likely to invite a torrent tracking company, it's "safer" than public torrents. But sometimes they'll get in from open registrations, or someone will give them their user account for whatever reason etc etc and you'll still get a notice.
You can also use a VPN, which is the most effective way to avoid being caught for multiple reasons. One, the VPN uses a different IP address than you so the IP they collect can't necessarily be traced back to you. Two, VPNs encrypt your traffic (unlike proxies) so you ISP itself wouldn't be able to confirm you're actually torrenting instead of, say, downloading a game on Steam or something. But despite being the most effective, you can still get caught if the VPN your using doesn't allow torrent and they receive a notice but they'll usually use cancel your account and that's it. There are torrent-friendly VPNs out there, though.
As to why you've never been "caught", there are a lot of reasons. As I mentioned, some ISPs don't bother with notifications at all and just ignore them, sometimes the tracking software can't verify the piece they're downloading from you is copyrighted, you might have your upload rate set to really low so on one is able to download as much from you etc etc. They also can't monitor the activity of the millions upon millions upon millions of people torrenting stuff, either, they usually prioritize the original uploader first (even though that's difficult to find).
The best way to not get caught is to simply not torrent and use other methods if you absolutely have to.