The trouble is that income/wealth inequality is often ignored in macroeconomic models. The math gets tough and an easy way to simplify the equations is to assume an equilibrium growth path. More complex models use a dynamic stochastic general equilibrium, but an equilibrium all the same.
Unfortunately, the key feature of rising inequality is that it may move the system into an unstable equilibrium. For example, if a handful of billionaires hang out together at Davos and get a bit of groupthink, they might decide to misallocate some capital (accidentally). If they make a mistake in a truly dramatic fashion, (investing heavily in Bitcoin, for example) they could evaporate a large chunk of the nation's wealth. When a handful of actors has an extreme influence on capital allocation, we've effectively become a command-economy instead of a free-market economy.