More stuff to break, worse failure modes, more work plan to install, almost universally more expensive, and 99% of people don't need water cooling to effectively and quietly cool whatever components they're using.
There's also a big case to be made that computers don't need to be anywhere near as powerful (read: power hungry) for what most people use them for but that's a whole different worm canning factory.
This is all just talking about AIO coolers. Custom water loops are even more daft for almost everyone.
More stuff to break - Pump failures AND coolant degradation. You're going to hear me complain about pumps a lot here.
Worse failure modes - Pump failures! You only get one pump and if it fails you better find out soon. Most systems should shut down before the coolant pressure gets too high but that shit sucks either way.
More work to install - Pretty straightforward, compatibility and finding room for everything stinks and getting it all put in stinks. Yes I see you out there saying you enjoy it, that's nice.
Almost universally more expensive - Are there water coolers that can outperform top class air coolers for under $100? I'm not aware of any but I don't exactly keep up to date.
Most people don't need it - Seriously, if you're not running a workstation with top end equipment, you can probably make due just fine with air cooling. For a short period of time you could exclude anything that wasn't "HDET" class but then everyone upped their power headroom for desktop chips again to get better benchmarks so you can probably get one that will overwhelm the best air cooler now. There's no reason anything made for normal games, even casual game dev or video workflow should really need it. The people who ARE finding things that require loading up >24 cores plus a GPU or whatever for their business can probably afford something with black rubber hoses that still needs flushed every 12 months at best.
But again why everyone needs a system that draws 400w+ at the wall just to play shooty bang bang 5 is beyond me.