Take that answer and repeat it for 1- the number of voters in your district. And its not just true for elected offices, party offices are often given to "those who show up".
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.
Going after incumbents is hard enough, going after an incumbent with their voters as a majority is impossible. You could spend millions on your campaign or you could instead burn the money and achieve the same results.
Not just in Texas. I filled out my ballot here in California and there were several local positions which were unopposed, or even multiple Republicans.