Agree with Kwabbit, Tekken Guy and Skill and Chance.
The wealthy and educate suburbs had some pretty massive swings from 2012 to 2016.
A major city going from 55%D to 75%D in one cycle like some rural counties went from 55%R to 75%R under Trump could mean over a million people changing sides. That's just not realistic.
Democratic communities are typically diverse and populated while GOP communities are homogenous and rural.
In the rural Midwest, some of these rural counties are 75% Whites w/o college degrees, so if that demographic is swinging massively, those counties will swing massively. Even in Franklin County, where Democrats have made strong gains, the potential Democratic coalition includes educated suburban Whites, Black and Hispanic voters, young urbanites, etc. A huge Democratic swing would require large gains among all of these groups.
The RGV is among the most homogenous areas in the country. If Trump was gaining massively among non-major metro Hispanics, the swing in the RGV would nearly be one-to-one. There are no countertrends to mute it.
Miami-Dade is diverse and Trump made gains among most populations, including Jewish voters and Caribbean voters, but the real cause was a massive gain among a group that forms about half the population: Latin American immigrants/refugees from socialist nations.
Democrats have huge swings too, it's just harder to notice on a county map. If you go on the precinct 2012-2020 swing setting for Kansas in DRA, you'll see big swings in 25k areas in Johnson County (which is about the size of these rural Midwest counties). I wish DRA could reupload 2012 data for other states. I bet you could find a 25k chunk on the Upper East Side in Manhattan that would've swung 50pts from 2012 to 2020.
To your point, in the last two presidential election cycles, Democrats have gained among the same demographic in college-educated Whites. So you have two 8pt swings. While Trump had big gains among WWC in 2016 and then big gains among Hispanics in 2020, two entirely different groups.
Kwabbit's analysis about demographics is nothing short of excellent, but he and Skill and Chance also touched upon another important point: urban counties are much larger than rural ones population wise, so an equivalent swing is pretty much unfeasible, or at least much harder to happen. If you divided up populous suburban and urban counties into many different counties so each had, say, 20,000-30,000 people (the size of many rural counties), some would have truly monstrous swings. Combining all of them makes it less visible. Which is why CONGRESSIONAL DISTRICTS are better - they're about equal in population, so it's a better comparison. When it comes to urban areas, sometimes congressional districts are actually smaller than counties, whereas with rural areas, you have to combine all the tiny rural counties as well as some small urban areas to get districts. GA06 is a good example. It swung over 30 points leftward from 2012 to 2020. There are only a few districts that swung as much to the right from 2012-2020.
