There are other standards than eyewitnesses for obtaining reliable testimony (2Tim. 3:15),
and even eyewitnesses can be wrong or give conflicting testimony, so I have no idea why you perseverate about that.
Do you believe the water, blood and Spirit were eyewitnesses? Does your belief make it true?
As for why agreement was expressed in that fashion, if it were expressed in the other way,
would you be asking why it was expressed in that way rather than in this fashion? And again,
why is this question so important IYO? Is whatever answer you arrive at inerrant?
The way it is expressed looks a lot like John was trying to express a truth that believers would recognize, but which would likely be missed by those who do not know the truth. The early church latched onto Platonic views by the early 2nd century, Hellenistic Judaism, which seems to have vanished very quickly after the 1st Jewish War also relied on Platonic views. Is this coincidence or is there a conceal connection. Much of what Paul and John say would fit with a concealed connection. As well as something that early Christianity wanted to keep concealed from the Romans.