Earlier quoted context omitted.
For the same reason words like "mansplaining" exist, presumably? I think outside perspectives can be useful, but sometimes they are just ignorant. Really depends on a) the perspective, and b) the intent
'Inside' perspectives can be equally useful or ignorant. The questions remains: why the distinction between inside/outside?
I see this frequently as an engineer: my pet peeve is the "can't we just..." from someone who has no idea how the system works. Occasionally they're correct that we could make a trivial change to make something work... But most times, that "just" is hand-waving away days/weeks of effort. On the other hand, when "can't we just ..." is uttered by someone else on the same team, they're usually correct that the change is indeed trivial.
In this case, "outside" vs "inside" is actually a good proxy for how informed or accurate the opinion actually is.
Another good example is the stereotypical "expert in a field who thinks their expertise trivially transfers to unrelated fields".
To put it more simply: the distinction exists because outsiders are very frequently blind to the internal complexity of something (a system, an idea, etc), but are still willing to confidently assert their ideas anyway, leading to a frequent association of "outsider" with "poorly-formed opinions".