Good talks, i agree someone can know everything thats written about a fish, but if a new species is discovered and brought into the hobby someone has to own it and set the standards for it before everyone else can have an idea of what to do.
I disagree.
When people discover fish, they typically do a lot. They get pictures (or drawings), and/or specimens, they document it's size, whether males look different from females, where they found it (river, lake, etc), often explain exactly where (surface, under rocks, in deep currents, etc), what food was in it's stomach, whether it was in groups, what other fish were nearby, what different behavior it had in daylight or evening, etc.
From all that, people can deduce enormous amounts of information about the probable temperatures, pH, diet, behavior, size of tank, water current, etc. it will need to have in a tank.
Yes, a lot more is gained from having it in a tank, because:
a) a lot of people observing will often add a lot of information compared to one isolated study, and
b) because fish tanks are unnatural environments where fish will exhibit unnatural behavior that one will rarely see in nature.
So of course, aberrant behavior generated in fish tanks is only going to be learned from having them in fish tanks. But it's certainly not true that no one had an idea of what to do with an RTC, giraffe catfish, pacu, angel fish, discus or piranha until someone put one in a tank.