Mostly in finance we assume that we have the equivalent of a standard dice. That is, while we assume we don’t know what number will come up next, we think that we know the distribution of numbers perfectly.
In fact the real situation is much more akin to throwing a dice where we have imperfect knowledge of what numbers are on the faces. They might be 1 to 6; but they also might be 1 to 5 with the 1 repeated; or 2 to 7; or something else entirely.
Worse, the numbers are changed by the malevolent hand of chance on a regular basis.
Not so often that we know nothing about the distribution, but often enough that we cannot be sure that the current market will be like the past.
(And I would add: sometimes the numbers on the die are being changed not by the malevolent hand of chance, but by the malevolent hand of a market participant who is smarter than you and can siphon your profits into their bank account.)