If you look at the red/blue split on election precinct levels, you'll see exactly why: the division is very distinct & consistent at the boundary between urban

If you look at the red/blue split on election precinct levels, you'll see exactly why: the division is very distinct & consistent at the boundary between urban and rural. There is no single clear line (we tried that in the 1860s, didn't go very well), and the geographic (as contrasted with populous) bulk of the so-marked areas sharply disagree with the portrayal. Example: New York is a "dark blue" state, but something like 90% of the land votes "red", noting that the bulk of the population is highly concentrated in a half-dozen cities; Georgia is a "bright red" state, with the city of Atlanta strongly disagreeing. The sharp red/blue division is a waterline surrounding islands of high population; rural folk often do smile at the notion of separating from "blue" areas, but the latter would soon find that severely problematic and will do anything to prevent it.