Don't most email clients make it pretty easy to create calendar events based on the content of emails?

You have to do that yourself but personally I prefer being in control of that anyway.

How long does it take you to add a flight to your calendar manually? A few seconds? Is your privacy worth those few seconds?

I took 140 flights last year. I made a larger number than that of online purchases. Adding / tracking all that stuff manually is a pure waste of time.

Use TripIt then. Looks like you’re nitpicking on small reasons.

Okay, so share my email with another company (fastmail + tripit now), and use a separate calendar (unless it integrates with google calendar?)

I trust gmail's security more than tripit, thanks. I think I'd be more worried about the world learning of my trip history if I used tripit than if I used gmail for that.

And also, the average user does not care. They don't want to think about it even to the point of researching a service, they get used to their phone telling them they have a flight the next day, and they want that to keep happening. They don't think about how it works and if they did, "oh no my privacy" is not something they'd think afterward.

What loss of privacy? In both cases the contents of your email and stored on a disk and read into memory for processing.