- They are loud
They can be loud, depending on the switches you get. However, some people feel that they benefit from it. The loud clicks are satisfying in some way. Maybe it's an ASMR thing for some people. For me, the sound feels like feedback.
- They are expensive
They can be. But so can regular keyboards. My first mechanical was like $40.
- The keys are massive and ugly
Unless you put low profile switches on your keyboard
- Massive travel time (So slow to type on)
Unless you put fast switches on your keyboard. The clickiest keys tend to be the slowest, but I don't type more than 80 words per minute anyway, and when I'm coding a lot less than that. But you can definitely get faster switches.
Here's another reason: it's fun. It's just fun. Sure maybe it's just cosplaying as a hacker, but it's still fun.
And most mechanicals can be remapped really easily, to allow custom workflows. Or layers. What I may potentially lose in words per minute, I gain by creating an intuitive key map that lets me reach the keys that I actually need day to day.