The USA's populist leader advocates that the existing arrangements are grossly unfair to the USA. The major focus of Trump's disquiet is China. As such, the USA is trying to level the playing field, pushing it to a rules-based regime. At the moment, countries self-declare their "developing" status; because there's nothing but upside to declaring yourself a developing nation there are many of them - so much so I'm surprised the USA hasn't declared itself a "developing country":
Special and differential treatment (S&D) entitles developing countries to longer time periods for implementing agreed commitments, measures to increase trading opportunities, and twice the amount of agricultural subsidies available to developed countries.
The USA position isn't that
countries classified as “high income” by the World Bank, OECD members or acceding members, G20 nations and any state accounting for 0.5 percent or more of world trade
aren't poor, or unpleasant to live in. What it is saying is that those countries are big, strong or rich enough that they don't need the easy set of WTO rules; let everybody else decide for themselves.
Source: U.S. drafts WTO reform to halt handouts for big and rich states, February 15, 2019.