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There are two reasons for that as far as I can see:

In past macroeconomics combined not just what we think of macroeconomics today but also a great deal of public economics which deals with inequality (one can see that just by looking at the writings of Keynes or Hayek).

However, nowadays macroeconomics don’t deal with distributional questions any more because these questions got subsumed into the subfield of public economics. If you check the public economics literature it is almost all about income and wealth taxation under different social welfare functions (like Ralwsian max min principle or utilitarianism or libertarianism/conservatism or respectively charitable libertarianism and conservatism which are similar to Ralwsian objective but instead of placing 0 welfare weight for anyone else then poorest person it still puts positive weights on those).

I think that the transition happened mainly because the models from public economics are mostly all micro models and they are rarely general equilibrium models, so by natural evolution partial equilibrium models simply didn’t got published much in macro so they developed their own niche and now even general equilibrium redistribution models get published in public economics literature (although this is just my conjecture I am young scholar myself so I did not witnessed the transition first hand but when you look at the literature this seems to be the reason).

Second reason why macro does not pay much attention to the inequality is that simply reducing inequality is small change compared to what economic growth can do for you.

Consider the following though experiment: you start at point $t$ where you have 1 rich person with 100e and 1 poor person with 50e now imagine you have two options either reduce inequality by redistribution to zero or not touching inequality but have 2% growth. In the first scenario the poor person will get 25e more which is good but in the second scenario just after $50t$ the income of the poor person is more than 100 that is more than originally the rich person had, and this increase is by nature exponential.

This should not be interpreted in a way that inequality does not matter but if you look at the “big picture”, which many would say is the point of macroeconomics, then the real prosperity and better life comes from economic growth not redistribution. Think of how the life would have been if our production would be at the medieval level and we would just be living in egalitarian society. However, this should under no way be interpreted as dismissing concerns about income inequality. They are valid and depending on your moral philosophy if you are Ralwsian or Utilitarian they are moral imperative, but from macro perspective it’s small change compared to great wealth that comes from growth. Hence assuming economists want to research the more valuable source of prosperity (even if just looking at the poor) then it makes sense they will look more at growth than inequality.