Serfs probably had a higher standard of living than cavemen, but that doesn't mean that the system of nobility and kings ruling over them was objectively correct, and should be maintained as the one true way to run a society.
The fact that billions have been lifted out of poverty in the last few decades is an important note and definitely deserves merit.
https://www.npr.org/sections/health-shots/2017/03/23/5210833...
https://avc.com/2016/08/elephant-chart/
(note that the green line marks the mean gain, at about 25%, not zero)
So the typical 1st-world person sees people on both sides of them gaining by leaps and bounds, while their own situation is stagnant. That is bound to cause some bitterness. The people below are now providing competition that pushes down the cost of labor, both via outsourcing and immigration.