Around 2004, I learned most of my code didn't run on real hardware, but ran on emulators. I found out why (the main reason: writing to video memory while the screen is drawing), and submitted patches to the SNES emulators of the time to work like real SNES hardware, but they were rejected because too many fan translations already relied on the old mistakes.
No one back then seemed to care about how accurate emulators were, so I set about writing my own SNES emulator with the goal of making it as perfect as possible, and it kind of spiraled out of control from there. Within a few years we were decapping chips to extract coprocessor firmware, I had to emulate the Game Boy for the Super Game Boy cartridge, the GBA CPU for this one Japanese chess game that had an ARM CPU in it, etc.
I guess I just really like the challenge, and never stopped adding more systems over time. The more systems I emulate, the more I already have most or all of the chips used in the system emulated. Eg ColecoVision took me an hour because I already had a Z80 core, a TMS9918 video core, and an SN76489 audio core. I can definitely see how MAME turned into what it is today now.
These days, the emulation is the easy part, and organizing all of this code (around 600,000 lines and counting), and getting all of these really unique pieces of hardware to all work in the same GUI, has become the challenging part. I have this really elaborate tree-view of the emulated systems to support zany things like the Genesis "Tower of Power", a complex scheduler that tracks timeslices in attoseconds, practically a full professional DSP stack for mixing multiple audio streams, Reed-Solomon decoding for CD systems, etc. I'm always learning new stuff, and there's always more to improve. I worry that I won't be able to wrap up higan in my lifetime.
There's not a lot of money in emulation, at most I've been offered $2500 for commercial licenses, but showing my hobby work to my employers landed me my last two jobs in software engineering, the latter of which is in Tokyo, Japan. Emulators literally got me halfway around the world. And I even got to work with the developers on Stephen Hawking's voice machine software at one point.
There's been some downsides, and I had a lot of maturing to do over the years, but on the whole, I wouldn't trade this hobby for anything.