Wealth isn't quite zero sum but it is adversarial. If someone has a lot of it they can use it against you. Use it to propagandize against your class interests, use it to manipulate the political environment to your detriment. The wealth may not be zero sum but the power it represents certainly can be.

"How many people has he '''made successful'''" is a fuzzy and already biased metric, how do you decide on that one? Why not "how many people would have had a better job if not for his company" or "how many liters of urine in soda bottles because of his policies?"

Wealth is concrete and measurable. People can differ on what exactly the consequences of wealth inequality are but I think nearly everyone will agree that it measures something worthwhile to know.