As usual a lot of good info on why to test why we don't "need" to test and what tests are just not really available to the average hobbyist. I think once you've been doing this a while like
RD.
,
duanes
tlindsey
and so many more, you shouldn't need a regular test as long as your husbandry skills are where they need to be (and you aren't lying to yourself). Newbs need to test on a regular basis. If your water source changes or you add or subtract fish you should test. I take my water to my lfs about once a month and have them test for me.
People with experience are more in tune with their tanks and can usually tell what's going on with just a glance.
There are tests that most of us do not do and probably should as
S
squint
mentioned if you want or need a fuller picture of what is going on. I think the average fish keeper probably doesn't need to test for it or should I say wouldn't even if they could. Lots of stuff is in place or can be to supposedly help negate those.
When I did salt I had pH probs, temp, redox, I used Hannah meters for nitrate, calcium, alk, po4, obviously I used a lab grade refractometer and used calibration fluid monthly to zero it. Multiple tds meters there is more I'm sure I've forgotten.
I think to answer the original question. It's people 1. don't know
2.don't want to spend the money on something they don't understand.
3. don't care to learn about it.
Someone else said it not everyone cares really what happens they "care" but if it dies I'll get another one. I see that so often in practice, in condolences, all the time. I think we all in reality kind of feel that. I'd like to say I don't, on the surface I don't, I feel in my heart I don't, but when a pet dies we get a new one. I thought when my staffie died I'd never get another dog again I loved her so much. We had a puppy before she died....I miss my staffie and I'm sure someday I'll own another. So in reality animals are replaceable, in a manner of speaking.