Because they're much friendlier to get started for the novice, most of the widespread examples seem extremely trivial to someone with experience.
Now- I'm DEFINITELY not an expert, probably barely mediocre with the EE fundamentals, but I did take a lot of EE and some robotics at Uni to get my degree in Computer Engineering. I am nothing more than a hobbyist but some of my cooler projects have been:
- Autonomous 4WD robot
- Quadcopter
- A USB nerf gun that shoots at the developer who broke the Jenkins build
- Used an electronic photo frame as a display to show happy birthday tweets and Facebook posts to my wife from all the friends and family on her birthday
- WiFi/Proximity garage door opener (before they were cool)
- Garage freezer monitor (after an accidental unplug that spoiled a season's worth of salmon and elk)
- A vending machine that let you pay with your work badge from 'points' you earned (this involved a Raspbery Pi and a surface tablet as well, but the Arduino was what actually controlled the vending machine)
I've done a ton of random home automation IoT gizmos, mostly just for the fun of it, and a lot of robot builds for the boys. But the reason I use a lot of Arduino is that they are pretty friendly to use, there's a lot of help when I'm stuck, and I often don't need more. When I do, sometimes I just add another Arduino (or 4) and use serial or SPI communications between them. I've got some PIC chips for stuff, and some other random chips laying around, but I get more done quickly with the Arduino.
Now, if I ever got on to an idea I wanted to make more permanent I would look elsewhere, but for a hobby, everything else provides more friction and ramp up in the few hours a month I can free up to work on stuff.
EDIT: I'll add I've gotten some ESP8266 chips/breakouts that I've gotten more used to and I may start using them in a lot of places I've previously used Arduino + WiFi because the WiFi breakouts use a LOT of the available I/O and are usually the most expensive part of a build. I think I have 2 or 3 that I reuse.