Most of the top tier wealth is driven by investments and in the wake of globalization (and "easy" money from CBs,) stocks and other asset classes have risen drastically. Given that the lower rungs in the world all have little to no investments compared to the "upper class" it's just basic math than people with investments will out perform those with none.
There will undoubtedly be a mean reversion on asset prices at some point in the future, which will lower the wealth gap as prices fall...but that might not be a good thing. What we really need, if we want to bridge the gap, imo, is teaching people to invest their money...and I mean in more than just a 401K and a house. If you look at actual living standards, even the middle and lower classes are in much better shape than they were in every previous decade. So we're making strides there. So we need to improve our education system (specifically for the lower class) and teach everyone to invest in the economy as a whole.