Writing a book that is a collection of other people's expertise / experiences / stories doesn't make you an expert.

Oh, it will certainly allow you to be perceived as an expert.

And it makes you a good collator of information, and, if you've been paying attention, a good student of your interviewees.

But you're not really an expert.

Not yet, anyway.

This isn't to say that you shouldn't write a book.

I'm just saying that it may make more sense to write a book about something you are personally good at, even if it's not necessarily a "marketable" or "popular" book.

If I want to learn from a known expert about their expertise, why would I learn it second hand from someone else?

Unless that person has something valuable to add.