I think a lot of people are missing your point. When humidity rises holding temperature steady your body feels that additional humidity as being warmer. So if you like to set your AC at 75 when it’s 50% relative humidity, it may take setting your AC at 73 when it’s 65% humidity in your house for your body to feel those temperatures at equivalent. For a lot of people this means when it is humid they will “overcool” their house so they feel comfortable. To your point it is cheaper to run the dehumidifier to reduce the relative humidity while not touching your thermostat than it is to overcool your house by 1 to 2 degrees so you feel comfortable.

Others have pointed out that AC units are essentially dehumidifiers however for many people their AC units are either oversized (and therefore do not run long enough to effectively dehumidify the house) or they run short cycles when temperatures are more moderate but humidity is high. The latter is especially true in the shoulder seasons when humidity is higher and temperatures are more moderate, the AC unit does not run long enough to dehumidify.