First of all, if you can't smell the uric acid and urea in piss, there's probably something wrong with your nose.

Piss sitting in a bowl for hours will become really stinky later. When leaving the body of a healthy person, piss can be considered sterile. Once it's exposed to air and sits in a bowl however, microorganisms will start living off of it and will reproduce, which alters it and often creates extra smells.

This has nothing to do with how much water you drink. It's just because of urine is how the body gets rid off waste. There are spores in the air and bacteria clinging to difficult to reach areas of the bowl, as well as living in the pipes beyond it. Those will reach the bowl and find some of the waste can still nourish them.

For real. I have a pissoir in my garage that i don't use too often. Once my dad was over and used it and apparently didn't flush. I went in there a day or two later and it was full of murky brown-ish water that smelled absolutely disgusting. I know the last time he drank water was probably in 2006, but still

...or they just live with someone who needs to be drinking more water.

Piss smells when it has been sitting for hours and hours. If your piss is totally clear, you are over hydrated. You don't need to be drinking an excessive amount of water unless you have particular needs. Normal urine should have a slight yellow tinge.

Just because you've gone nose blind to your piss filled toilet, doesn't mean that people you have over will be. I had a grandfather that absolutely never flushed for urine and his place smelled disgusting whenever I would go for a visit.