I think it's because people's wealth is really hard to estimate. The border between people's assets and their companies', foundations' and what not are usually blurry enough that you would get a graph that shows no insight.
Although income is not a great KPI, at least there are reliable methods to figure it out, or at least make estimates with a quite acceptable precision.
Say someone could have a small business that takes off and retires after a few years with a reasonable retirement fund. This doesn't fit the mold of a hoarding 1%-er.
It seems like discouraging this kind of income spike would actually decrease social mobility.
"“The probability of ending where you start has gone up, and the probability of moving up from where you start has gone down,” Carr said. For instance, the chance that someone starting in the bottom 10 percent would move above the 40th percentile decreased by 16 percent. The chance that someone starting in the middle of the earnings distribution would reach one of the top two earnings deciles decreased by 20 percent. Yet people who started in the seventh decile are 12 percent more likely to end up in the fifth or sixth decile—a drop in earnings—than they used to be."