County elections in South Carolina aren't separate, but municipal elections are.
Is there a reason for that?
Statewide elections are run by the State Election Commission in conjunction with the County Election Commissions. State and County offices and election dates are set by State law, not County law. (This is a holdover from the pre-
Reynolds v. Sims days when each county's delegation in the General Assembly also served as that county's council.)
By contrast, the municipalities run the elections and set the election dates themselves. Some do choose to set the date for the general election, but most do not. (The town I live in uses the Tuesday after the first Monday of November, but in odd-numbered years instead of the even-numbered years of the general elections.)