I don't know which Spotlight search you've been using but it certainly has not been consistent and reliable in years.

And I don't find manually searching for emails to find something like flight dates that easy. I've flown to/from Mexico city 6 times in the past couple years, and the airline tends to send like 12 emails pertaining to every trip I book, so asking Siri is much faster.

Not to mention these are tasks I'll be able to do in winter while wearing gloves.