Wealth can be created, and not just taken from someone else.
As evidence, look at this chart of per-capita world GDP:
(years on the bottom, dollars on the left)
This is the average GDP, per person, of an entire planet. If increasing wealth can only be the result of "transfers" from other people, where did this all come from? Saturn?
It is imperative for the Left to paint wealth as being unearned, or somehow "stolen" or forcibly "transferred" from people who don't have any money. This gives the Left the moral authority to loot this wealth for their own purposes.
In the real world, the forcible transfers go the other way.