Why do technical interviews seem so intimidating?

You are a complex good.

Neither you nor the firms assessing you, generally, know how to sell, or how to buy, what it is you're offering.

That said, more practice at interviewing generally improves your performance, though it may have little to do with actually improving the outcome of the process in terms of offer-to-opportunity matching.

But I try to rationalise that. Throughout my career, I've worked with people smarter than me. I'm not intimidated by them, so why should I be during an interview?

With that in mind, I assume they know more until proven otherwise. And if they do know more, I treat them the same I would outside of the interview.

You need to practice this part then. It doesn't take a huge amount of time when you generally know it already and it's guaranteed to help in most interviews. It's easy to be confident when you've seen the exact same questions before. There's only so much you can ask without being obscure.

The reason I suggest it is because knowledge and insight into the hiring process might make the interviews less stressful.

Good luck.

In the ideal world, your hard-earned knowledge and experience should prepare you for the challenges in the role you're applying for, but sadly most interviews test for some kind of coding ninjas that spend their days writing optimized merge sort implementations before lunch and building high-performing distributed caches in the afternoon, all day every day. Mind you, none of those problems are particularly difficult, just let's not kid ourselves that they represent the kind of daily challenges you're going to face even at places like Google or Facebook (and especially not under the same constraints as the interviews). The fact that Cracking The Coding Interview is considered a must-read before interviewing is a bright indicator of how bad things have gotten.