* 30+ years of typing (8+ hours a day) has caused some arthritis in my fingers. By going with a 'custom' mechanical keyboard i can pick switches (the parts that go click), keycaps (the parts you push down on), and the gasket (the part that cushions the blow when you click).
* They are super nerdy and fun to tweak exactly how you want to feel, look, and sound.
I have a small collection of keyboards (priced range $50 - $600)... but the keyboard i use 99% of the time is a Keycron Q1.
Its main competitor (that i have at my office i no longer work from) is the Glorious GMMK Pro:
They are what Keyboard freaks would call 'entry level' (~$200) and most people don't need more.