Could anyone explain why trade secrets are something you can sue over if no NDAs or similar have been signed?

A. (the sarcastic answer)

The USPTO explains the reasoning here:

https://www.uspto.gov/patents-getting-started/international-...

As that page makes clear, it's now a federal offense to misappropriate trade secrets because:

"As a member of the World Trade Organization (WTO) and a party to the Agreement on Trade Related Aspects of Intellectual-Property Rights (TRIPS), the United States is obligated to provide trade secret protection. Article 39 paragraph 2 requires member nations to provide a means for protecting information that is secret, commercially valuable because it is secret, and subject to reasonable steps to keep it secret. The Defend Trade Secrets Act of 2016 created federal civil cause of action, strengthening U.S. trade secret protection, with a choice for the parties between localized disputes under state laws or disputes under federal law, heard in federal courts. While state laws differ, there is similarity among the laws because almost all states have adopted some form of the Uniform Trade Secrets Act. "

If for some reason you need a deeper explanation (although I'm already getting a bit suspicious as to what legitimate reason you might or might not have), that page links to a helpful Youtube video:

https://www.youtube.com/watch?v=1dXA5A4l0Rg

While unfortunate that the USPTO has no way to distribute video information without relying on its industry partners to help get the message out[1], the video clarifies that "The failure to identify and protect trade secrets can result in a loss of competitive advantage, loss of core business technologies, and reduced profitability. In many countries the importance of trade secrets is reflected in laws and regulations that protect against trade secret theft because of the impact it can have on the economic vitality of businesses."

Egad, the specter of "reduced profitability" and the loss of "economic vitality"! Underlying this important message are a series of cartoons subconsciously illustrating the principle that without federal protection of trade secrets, the recipe for the "World's Best Cookie's" would quickly be stolen by foreigners with funny looking facial hair, and then we would no longer have cookies.

Any other questions?
</sarcasm>

B. (the non-sarcastic answer)

Because businesses with lots of money gave some of that money to politicians to create laws allowing them to use the muscle of the federal government to preserve the competitive advantages they already have. Trade secrets are to patents roughly as the DMCA is to copyright: they add a layer of federal felony to what would otherwise be a civil dispute.

[1] Even the interstitial makes me gag: "You are leaving USPTO.gov. We provide this link to an outside website because it has information that may be of interest to users. The USPTO does not necessarily endorse the views or facts presented on this site. The USPTO does not endorse any commercial products that may be advertised or available on this site." They make the video, post it on an external website, and then say that are linking to an external website because it might be of interest to users? And what compelled them to say "does not necessarily endorse"? You really should watch the video, though, as its "smarminess" is almost beyond words.