Look at what percentage of the vote those Republicans won in the last election, and if it's over 60%, I can almost guarantee there's not a Democrat alive that can win that office. Doesn't make any sense for a Dem to run in a race they can't win, as it would take resources away from Dems in competitive races.
Oh sure, totally understandable. I grew up in a mid-Atlantic state during the 80s & 90s, where even the Democrats would've been Republicans anywhere else. Tons of GOP incumbents never had opponents. It gradually got much better but it takes time and a change in demographics to make those changes, not Don Quixotic campaigns that are destined to lose.