Other reasons people like them usually are about customizability. If you think of it as being roughly similar to how some people can be about noodling with cars, you won't be too far off-base.
- With the right firmware, they are fully customizable - you can remap things how you like, create macros, whatever, and config is stored on the board.
- You can customize the switches - there are a bewildering array of switch options. People also spend lots of time customizing switches in various ways to change how they feel.
- Layouts - keyboard nerds have produced a huge array of different layouts.
- Looks - you can buy your own keycaps. Those, too, come in a wide array of profiles and appearances. Some people make really nice boards out of metal, wood, sometimes odder things (I heard about someone trying to make cases out of Corian at one point). If you like that sort of thing, you can have them covered in blinky LEDs. There's a whole tiny art scene making keycap-sculptures that go for a ton of money, relatively speaking.
And that's before you get to the crazy stuff that leads to people paying $1k+ on a keyboard.
Really, it is like anything else - people do a lot of things as hobbies that look odd from outside.
But seriously, I never understood the craze for these clunky things. Almost all devs I know use them and most of them have issues with their arms/hands.