Like most professional investors, I don't tend to sign NDAs prior to a formal engagement with a company.

Why? There are a number of reasons that include...

Your idea is not unique. What's unique is the 10,000 decisions you will make to execute your idea between now and success and beyond
Even if people have the exact same idea, there are likely many, many ways to execute the idea producing many different kinds of companies each with a different niche/focus/go-to-market etc. Again, here, the execution is what matters.
Rather than people stealing your idea, it's more likely that you need to drag them kicking and screaming to it. The best people are too busy doing their own things to be going around taking other people's ideas.
Since I meet with a lot of companies in my role as investor and advisor, it's likely I've heard versions of your idea before and I'll hear many more versions in the future. If I choose to engage with these companies I could be accused of sharing your idea which - in fact - did not happen. This exposes me to unnecessary risk
It adds paperwork and friction to everything
It filters out Founders/companies that don't know what they're doing and refuse to be coached.

That being said, I will, of course, always act in a professional manner and treat the information I get as confidential and use it only in the best interests of the founders and the company.

Also, my formal engagement contract does have a confidentiality clause since, in those cases, I will be digging much deeper into execution details.