Why do gurus use different websites for each product?

"Never put all your products in one basket" err.. eggs on one site.. well

There could be lots of reasons to separate them. If a site does well in Google and then gets hit with a penalty for some reason, all the products won't go down. Then again, having them all on the same site they could draw influence from one another making it a more authoritative site. Could be they want to keep products separate to later sell the business, etc..

Well, I'm not a "guru" by any means. But this method may also be necessary if you have segmented lists or are appealing to different niches. For example, I am promoting many dozens of niches and each niche has as many as 300 product websites. It's a marketing method, and not exclusive only to gurus. Testing is your friend.

Two possibilities:
They want a search optimized domain name that matches the product, which also may receive some type-in traffic a sub-directory wouldn't get.
They treat each product as a self contained business that can be sold to a different guru, lock-stock & site.
Of course they may get in the habit of insulating their ventures operated under pen names from being discovered by their competitor.

a domain name is 7$ per year, i doubt they are looking to save money anyway... and as the above pesters mentioned it is safer, less conflicts between niches and just looks much better...

Just because they are using different domain names, doesn't mean they aren't on the same server...

I routinely have several domains on a single server, partly for convenience, partly because I'm cheap. Why should I pay a hosting fee for each domain, unless they have enough traffic to earn a dedicated server from their sales?

Using subfolders to host different domains is a common practice, and I've discovered more than a few just with nslookup on a PC. Do nslookup domainname.com from a command box, and note the IP address. Repeat with other domains the same guru owns. If you see the same domain name pop up, said guru is probably hosting several domains from the same server. If you've got some extra time you can try to figute out what folder names the guru used to host his other domains. Typically isn't worth any time, because you won't see anything that you wouldn't see from the regular domain name that points to the subfolder. Interesting, just not useful.

Another reason for multiple domain names might be protecting your files from thievery and casual download, though this is something that is easy to do if you have a good "tech guy" or know the ropes, relative to securing your files. If you don't, you may wish to look at my WSO (first link in my signature) about protecting yourself.

Not "Putting all your eggs in one basket" is often given for having different domain names, when in truth, there will normally be several domain names on a single server. Most savvy folks have multiple hosting accounts. Personally, I have my site files on the "production server" with one hosting provider, and another copy on a "backup server" with another hosting provider. That way, if my primary provider has a bad day and my site is unavailable, all I need to do is go to my name host and change the nameserver to the alternate provider. This was a "last ditch" correction 10 years ago, when it took 72 hours for name server changes to propagate, but these days, I've seen down time from discovery of a dead server and a change of nameserver, with the backup host live from my location within as little as 5 minutes, and halfway around the world in an hour. My worst time to recovery has been significantly less than an hour. Much better than waiting for your primary hosting provider to move to another server and recover backups, or whatever they do to get back online.

Bottom line, the big reason for different domain names is product recognition, and to a certain extent exclusivity. Next on the list might be SEO, though I've seen sites run as subfolder off of main site do just as well in search engine rankings as individual domain names.

For some products, you can actually beat one domain against the other (compete with yourself) and increase overall traffic and sales. If you use different owner pseudonyms, some folks never catch on to the game.

I agree with the above on different sites. Rick made a lot of good points.
I prefer separate sites myself. I'm setting up our first launch site with a partner and we're using a launch theme with specific plugins and specific timing for the funnel, etc.
Our next program won't be set up as a launch and will use a different theme and some different plugins. Since they are all set up differently and would need individual wordpress installs anyway I'd set up different sites to take advantage of keywords in the domain names,etc.
But they may be on a shared server on the same host unless I specifically need different hosting and IP's.
I just flat out like the idea of different products on different domains mostly for protection in case something bad happens to one site the rest don't get penalized.

It makes it easier from an SEO and marketing perspective. And also if they ever want to sell the business as well.

It makes things more focused.. one solution for one problem, lazer targeted.

I do both.. I have one 'main' website, and a bunch of smaller, more focused sites & lists.