Asset Piracy: How Common Is It for Creators?

Very common. You can even easily find pirated assets on reputable marketplaces, like cgt, or ts. The more popular and higher quality your assets are, the more chances that they will be pirated sooner rather than later.

If you find your stolen assets here on cgt, it's easy - you just report to the support and assets and possibly the thief will be taken down. If you find them elsewhere, especially in places like telegram, you're basically out of luck. You can try to fill takedowns and if the site is semi decent, you may even achieve asset removal, but it's just a matter of time when it will reappear in ten more places. As you correctly guessed by yourself - it's just inevitable part of being creator. Since some time i prefer not to look for my models online - what you don't know doesn't hurt you.

If your assets are pirated it means you are good at what you do :D. People pirating it would not buy it anyway. On a brighter side you get some kind of free marketing :D

It is worth your time to directly contact the supposed author/pirate on those websites and threaten then with a DCMA takedownn or other legal action if they don't remove your pirated product. I've found my water models on reputable sites such as Artstation Marketplace, as well as on more sketchy, less reputable sites. In all cases, I filed a DCMA take-down request with the website, and followed it up with a direct contact with the pirate. In most cases, the pirate removed my models before the website owners/managers responded. This is particularly good to do because some of those pirates don't want to get into trouble on those brokering sites, so they will deleted your product if you protest. Also, for some sites such as Artstation Marketplace, the process of filing a DCMA take-down request is unbelievably complicated and difficult to undertake. The process of registering a takedown request is difficult in its own right, and then you must go through the work of proving that the assets belong to you, of course. So, if you can confront the pirate and get them to remove your product it is far more expedient.

I sell 3D printing files. ​​And some people steal... my renders. The site looks like a scam store, a render of my product and it says that they are selling an irl copy of it. I'm 90% sure they didn't even buy the file.
After that I started adding watermarks to all renders.

True, some of these guys, apparently, steal in an automated mode. That's why they add my nickname to the product name. On the one hand, free advertising. On the other, I'm waiting for messages from people who haven't received the goods.